Steel material

ABSTRACT

Provided is a steel material which can achieve excellent fatigue strength even when a carburized steel component is produced by welding before carburizing treatment. The steel material has a chemical composition containing: in mass %, C: 0.09 to 0.16%, Si: 0.01 to 0.50%, Mn: 0.40 to 0.60%, P: 0.030% or less, S: 0.025% or less, Cr: 0.90 to 2.00%, Mo: 0.10 to 0.40%, Al: 0.005 to 0.030%, Ti: 0.010 to less than 0.050%, Nb: 0.010 to 0.030%, N: 0.0080% or less, O: 0.0030% or less, B: 0.0003 to 0.0030%, Ca: 0.0005 to 0.0050%, and the balance: Fe and impurities, and satisfying Formula (1) to Formula (3) according to the description. In a cross section parallel to an axial direction of the steel material, an amount of Mn sulfide is 70.0 pieces/mm2 or less, and an amount of oxide is 25.0 pieces/mm2 or less.

TECHNICAL FIELD

The present invention relates to a steel material, and more specifically to a steel material which is used as a starting material for carburized steel components.

BACKGROUND ART

The steel material used as the starting material for mechanical structural components to be carburized (carburized steel components) generally contains Mn, Cr, Mo, Ni and the like. The steel material used as the starting material for carburized steel components is produced by casting, forging, rolling, or the like. Then, the carburized steel component is produced by, for example, the following method. The above described steel material is forged. The forged steel material is subjected to cutting work to produce an intermediate member. The intermediate member is subjected to carburizing treatment. Through the production process described above, a carburized steel component including a carburized layer which is a hardened layer of a near surface portion, and a core portion which is a base metal that is not affected by the carburizing treatment is produced.

Of the costs of producing a carburized steel component, the cost of cutting work is very large. In addition, since the cutting work produces a large amount of chips, which reduces the yield. For this reason, a technique for replacing cutting work with forging has been studied. The forging method can be roughly classified into hot forging, warm forging, and cold forging. Warm forging is carried out in a temperature range lower than that of hot forging. Therefore, in warm forging, a smaller amount of scale is generated as compared in hot forging, and dimensional accuracy is improved than in hot forging. On the other hand, in cold forging, no scale is generated and the dimensional accuracy is equivalent to that of cutting work. Therefore, a technique of performing rough machining by hot forging and thereafter finishing work by cold forging, a technique of performing warm forging and thereafter performing minor cutting as finishing work, and a technique of forming only by cold forging, etc. have been studied. However, when cutting work is replaced with warm forging or cold forging, if the deformation resistance of the steel material is large, the interfacial pressure applied to the die increases and the life of the press tooling decreases. Therefore, a cost advantage by replacing the cutting work with forging is reduced. Moreover, when forming into a complicated shape, there arises a problem such as cracking in a region which is subjected to a large amount of working. Therefore, when the cutting work is replaced with forging, it is required to soften the steel material or improve the critical upsetting ratio of the steel material. When the steel material used as the starting material for a carburized steel component is an as-rolled member, the critical upsetting ratio of the steel material can be increased by performing spheroidizing heat treatment.

Patent Literature 1 proposes a carburizing steel which exhibits smaller deformation resistance and a larger critical upsetting ratio during cold forging than those of a conventional steel at the stage of carburizing steel before carburizing treatment, and further has equal levels of hardened layer and core portion hardness to that of a conventional steel after carburizing treatment. The carburizing steel according to Patent Literature 1 has a chemical composition consisting of: in mass %, C: 0.07% to 0.13%, Si: 0.0001% to 0.50%, Mn: 0.0001% to 0.80%, S: 0.0001% to 0.100%, Cr: more than 1.30% to 5.00%, B: 0.0005% to 0.0100%, Al: 0.0001% to 1.0%, and Ti: 0.010% to 0.10%, with limitations of N: 0.0080% or less, P: 0.050% or less, and O: 0.0030% or less, and with the balance being Fe and unavoidable impurities, wherein the content indicated by mass % of each element in the chemical composition simultaneously satisfies Formula (1) as a hardness indicator, Formula (2) as a hardenability indicator, and Formula (3) as a TiC precipitation amount indicator, as shown below.

0.10<C+0.194×Si+0.065×Mn+0.012×Cr+0.078×Al<0.235  (Formula 1),

7.5<(0.7×Si+1)×(5.1×Mn+1)×(2.16×Cr+1)<44  (Formula 2), and

0.004<Ti—N×(48/14)<0.030  (Formula 3).

Patent Literature 1 states that this carburizing steel can increase the critical upsetting ratio during cold forging by having the above described chemical composition, and can achieve equal level of hardened layer and steel portion hardness to that of a conventional steel.

CITATION LIST Patent Literature

Patent Literature 1: International Application Publication No. WO2012/108460

SUMMARY OF INVENTION Technical Problem

By the way, a plurality of carburized steel components are used for mechanical structural components used in an automobile. For example, carburized steel components are also used in the variable diameter pulley of a continuously variable transmission (CVT). As described above, large-size carburized steel components typified by the variable diameter pulley are produced by performing hot forging and thereafter cutting work. Accordingly, even for large-size carburized steel components, a technique for replacing cutting work with forging has been studied. However, if a large-size steel material is to be formed by cold forging, excessive load will be applied to the cold forging mill. Therefore, when a large carburized steel component is formed by cold forging, a technique for producing a large-sized carburized steel component by forming a plurality of members by cold forging, and thereafter joining these plurality of members by welding such as friction joining or laser joining, and further subjecting the joined steel members to carburizing treatment has been studied.

When a carburized steel component is produced by welding in this way, the fatigue strength (fatigue strength of weld-joint) of the carburized steel component which is a joined member is required.

It is an object of the present disclosure to provide a steel material which can achieve excellent fatigue strength after carburizing treatment even when welding is performed.

Solution to Problem

A steel material according to the present disclosure containing: in mass %,

C: 0.09 to 0.16%,

Si: 0.01 to 0.50%.

Mn: 0.40 to 0.60%,

P: 0.030% or less,

S: 0.025% or less,

Cr: 0.90 to 2.00%.

Mo: 0.10 to 0.40%,

Al: 0.005 to 0.030%,

Ti: 0.010 to less than 0.050%,

Nb: 0.010 to 0.030%,

N: 0.0080% or less,

O: 0.0030% or less,

B: 0.0003 to 0.0030%, and

Ca: 0.0005 to 0.0050%,

with the balance being Fe and impurities, and

the steel material satisfying Formula (1) to Formula (3), wherein

in a cross section parallel to an axial direction of the steel material, an amount of Mn sulfide having, in mass %, a Mn content of 10.0% or more, a S content of 10.0% or more, and an O content of less than 10.0% is 70.0 pieces/mm² or less, and an amount of oxide having an O content of 10.0% or more is 25.0 pieces/mm¹ or less:

0.140<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235  (1)

13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0   (2)

0.004<Ti—N×(48/14)<0.030  (3)

where, each element symbol of the Formulae (1) to (3) is substituted by the content (mass %) of the corresponding element, and if the corresponding element is not contained, the element symbol is substituted by “0”.

Advantageous Effects of Invention

The steel material according to the present disclosure can achieve excellent fatigue strength in a carburized steel component after carburizing treatment even when welding is performed.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a cross sectional view perpendicular to a longitudinal direction of a steel material of the present embodiment.

FIG. 2 is a schematic diagram to illustrate a sample collection position in the microstructure observation of the steel material of the present embodiment.

FIG. 3 is a schematic diagram to illustrate a sample collection position when Mn sulfide and oxide in the present embodiment are measured.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a steel material according to the present embodiment, which is to be used as the starting material for carburized steel components, will be described.

The present inventors have conducted study for obtaining excellent characteristics (improvements of effective hardened layer depth and core portion hardness) of the carburized steel component after the carburizing treatment in the steel material which is used as the starting material for carburized steel components. As a result, the present inventors obtained the following findings (a) to (f).

(a) As the C content decreases, it becomes easier to obtain a softer steel material before cold forging. However, if the C content is too low, it is difficult to achieve a carburized steel component after carburizing treatment having a fatigue strength equivalent to that of a conventional steel material having a C content of about 0.20% (for example, SCR420 specified in JIS G4052 (2008)). In order to obtain hardness of the core portion required for carburized steel components, there is an appropriate lower limit of the C content.

(b) To obtain as large an effective hardened layer depth and core portion hardness as possible at a low C content in a carburized steel component, it is preferable to increase the martensite fraction in the microstructure of the core portion of the carburized steel component.

(c) To increase the martensite fraction of the microstructure in the core portion of a carburized steel component, it is effective to contain the contents of alloying elements (hardenability improving elements) that improve the hardenability of steel, such as Mn, Cr, Mo, Ni, etc. so as to satisfy Formula (2) of the above described hardenability indicator.

13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0   (2)

(d) However, when the contents of the above described hardenability improving elements increase, the hardenability improving elements dissolve in and strengthen ferrite. In this case, the hardness of the steel material will increase. For that reason, it is arranged such that B is contained, which can improve the hardenability of the steel material while suppressing the increase in hardness of ferrite, and that the contents of C and the hardenability improving elements satisfy Formula (1) of the above described hardness indicator.

0.140<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235  (1)

(e) To stably obtain the hardenability improving effect of B, it is necessary to secure sufficient solute B in the steel material during the carburizing treatment. Therefore, the steel material contains Ti. In this case, most of N in the steel material is immobilized as TiN during carburizing treatment. As a result. B combines with N, thus suppressing them from precipitating as BN, and a sufficient amount of solute B can be secured in the steel material. To effectively obtain this effect, it is preferable to contain Ti so as to be stoichiometrically excessive with respect to the N content. Further, to prevent abnormal grain growth of austenite grains during carburizing treatment, TiC is finely dispersed and precipitated in the microstructure. To secure a sufficient amount of solute B and make TiC finely dispersed and precipitated, it is arranged such that the contents of Ti and N satisfy the Formula (3) of the above described TiC precipitation amount indicator.

0.004<Ti—N×(48/14)<0.030  (3)

(f) Boron (B) effectively enhances the hardenability of the core portion of the carburized steel component. However, when gas carburizing by a gas-converter method is performed, the effect of improving hardenability due to the inclusion of B is low in the carburized layer which is a near surface portion of the carburized steel component. This is because, nitrogen invades from the surface of the steel component during the carburizing treatment, and combines with solute B and precipitates as BN so that the amount of the solute B is reduced. Therefore, in order to ensure hardenability in the carburized layer, which is a near surface portion of the carburized steel component, it is effective to satisfy Formula (2) of the hardenability indicator in (c) described above.

The present inventors further examined, in the steel material of the present embodiment, the fatigue strength (fatigue strength of weld-joint) of a carburized steel component produced by carburizing treatment after welding. As a result, they have found that if the following specifications are satisfied regarding inclusions in a cross section parallel to the longitudinal direction of the steel material (that is, the axial direction of the steel material), the fatigue strength (fatigue strength of weld-joint) in the carburized steel component, which is produced by carburizing treatment after being welded, increases.

-   -   (I) The amount of Mn sulfide having, in mass %, a Mn content of         10.0% or more, a S content of 10.0% or more, and an O content of         less than 10.0% is limited to 70.0 pieces/mm² or less.     -   (II) The amount of oxide having, in mass %, an O content of         10.0% or more is limited to 25.0 pieces/mm² or less.

This point will be described in detail below.

In the steel material having the chemical composition of the present embodiment, Mn sulfide and oxide are present in the steel material. Here, Mn sulfide and oxide are defined as follows in the present description.

Mn sulfide: An inclusion having a Mn content of 10.0% or more, a S content of 10.0% or more, and an O content of less than 10.0% with the supposition that mass % of the inclusion is 100%.

Oxide: An inclusion having an oxygen content of 10.0% or more in mass % with the supposition that a mass % of the inclusion is 100%.

Note that in the present description, among inclusions, an inclusion containing, in mass %, 10.0% or more of S, 10.0% or more of Mn, and 10.0% or more of O is included in “oxide”, not in “Mn sulfide”.

When a carburized steel component is produced by performing welding typified by friction welding, laser joining, etc., and thereafter performing carburizing treatment on a steel material, a HAZ region exists in the carburized steel component. The strength of the HAZ region may be lower than that of other regions. Therefore, in the present embodiment, inclusions are reduced as much as possible to secure the strength of the HAZ region. In this embodiment, as described in (I) and (II) above, the numbers of pieces of Mn sulfide and oxide that occupy most of the inclusions in steel are reduced. In this case, it is possible to secure the strength of the HAZ region, and consequently increase the fatigue strength of the carburized steel component.

The steel material of the present embodiment, which has been completed based on the findings described so far has the following configuration.

[1]

A steel material containing: in mass %,

C: 0.09 to 0.16%,

Si: 0.01 to 0.50%.

Mn: 0.40 to 0.60%,

P: 0.030% or less,

S: 0.025% or less,

Cr: 0.90 to 2.00%,

Mo: 0.10 to 0.40%,

Al: 0.005 to 0.030%,

Ti: 0.010 to less than 0.050%.

Nb: 0.010 to 0.030%,

N: 0.0080% or less,

O: 0.0030% or less,

B: 0.0003 to 0.0030%, and

Ca: 0.0005 to 0.0050%,

with the balance being Fe and impurities, and

the steel material satisfying Formula (1) to Formula (3), wherein

in a cross section parallel to an axial direction of the steel material, an amount of Mn sulfide having, in mass %, a Mn content of 10.0% or more, a S content of 10.0% or more, and an O content of less than 10.0% is 70.0 pieces/mm² or less, and an amount of oxide having an O content of 10.0% or more is 25.0 pieces/mm² or less:

0.140<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235  (1)

13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0   (2)

0.004<Ti—N×(48114)<0.030  (3)

where, each element symbol of the Formulae (1) to (3) is substituted by the content (mass %) of the corresponding element, and if the corresponding element is not contained, the element symbol is substituted by “0”.

[2]

The steel material according to [1], wherein

when a radius in a cross section perpendicular to an axial direction of the steel material is defined as R (mm), in the microstructure in the cross section perpendicular to the axial direction of the steel material, an area fraction of bainite in at least an outer layer region from a surface to 0.1R depth is 95.0% or more.

[3]

The steel material according to [1], wherein

when a radius in a cross section perpendicular to an axial direction of the steel material is defined as R (mm), in the microstructure in the cross section perpendicular to the axial direction of the steel material, at least an outer layer region from the surface to 0.1R depth is composed of ferrite and cementite, and a spheroidization ratio of the cementite in the outer layer region is 90.0% or more.

[4]

The steel material according to any one of [1] to [3], containing in place of part of Fe,

one or more elements selected from the group consisting of: in mass %,

Cu: 0.50% or less,

Ni: 0.30% or less, and

V: 0.10% or less.

Hereinafter, the steel material according to the present embodiment will be described. Unless otherwise specified. “%” relating to the chemical composition means mass %.

[Chemical composition]

The steel material of the present embodiment is a material for carburized steel components. The steel material of the present embodiment is cold forged and thereafter carburized to obtain a carburized steel component. The chemical composition of the steel material of the present embodiment contains the following elements.

C: 0.09 to 0.16%

Carbon (C) improves hardenability of steel material, and increases the hardness of a core portion of a carburized steel component including a carburized layer and a core portion. If the C content is less than 0.09%, the hardness of the core portion of the carburized steel component decreases even if the contents of other elements are within the range of the present embodiment. On the other hand, if the C content is more than 0.16%, the hardness of the steel material before cold forging remarkably increases, and the critical upsetting ratio will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the C content is 0.09 to 0.16%. Note that the C content of a steel material which is used as the starting material of a conventional carburized steel component is about 0.20%. Therefore, the C content of the steel material of the present embodiment is lower than that of a conventional steel material. The lower limit of the C content is preferably 0.10%, and more preferably 0.11%. The upper limit of the C content is preferably 0.15%, and more preferably 0.14%.

Si: 0.01 to 0.50%

Silicon (Si) increases a temper softening resistance of a carburized steel component, thereby increasing the surface fatigue strength of the carburized steel component. If the Si content is less than 0.01%, the above described effect cannot be achieved even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Si content is more than 0.50%, the hardness of the steel material before cold forging increases and the critical upsetting ratio decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the Si content is 0.01 to 0.50%. When emphasis is placed on the surface fatigue strength of the carbunzed steel component, the lower limit of the Si content is preferably 0.02%. When emphasis is placed on the improvement of the critical upsetting ratio of the carburized steel component, the upper limit of the Si content is preferably 0.48%, and more preferably 0.46%.

Mn: 0.40 to 0.60%

Manganese (Mn) improves the hardenability of steel material, and increases the strength of the core portion of a carburized steel component. If the Mn content is less than 0.40%, this effect cannot be achieved sufficiently even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Mn content is more than 0.60%, the hardness of steel material before forging increases and thereby the critical upsetting ratio decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the Mn content is 0.40 to 0.60%. The lower limit of the Mn content is preferably 0.42%, and more preferably 0.44%. The upper limit of the Mn content is preferably 0.58%, and more preferably 0.56%.

P: 0.030% or less

Phosphorous (P) is an unavoidably contained impurity. That is, the P content is more than 0%. P segregates at austenite grain boundaries and embrittles the prior-austenite grain boundaries, causing intergranular cracking. Therefore, the P content is 0.030% or less. The upper limit of the P content is preferably 0.026%, and more preferably 0.024%. The P content is preferably as low as possible. However, if the P content is decreased to the limit, the productivity deteriorates and the production cost increases. Therefore, the lower limit of the P content is preferably 0.001% in normal operation.

S: 0.025% or less

Sulfur (S) is unavoidably contained. That is, the S content is more than 0%. S combines with Mn to form MnS, thereby improving the machinability of steel material. If the S content is more than 0%, this effect can be achieved to some extent. On the other hand, if the S content is more than 0.025%, coarse MnS is generated so that cracking becomes likely to occur during forging and the critical upsetting ratio of steel material decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the S content is 0.025% or less. The upper limit of the S content is preferably 0.022%, and more preferably 0.020%. When the machinability is improved more effectively, the lower limit of the S content is preferably 0.001%, more preferably 0.002%, and further preferably 0.003%.

Cr: 0.90 to 2.00%

Chromium (Cr) improves the hardenability of steel material, and increases the strength of the core portion of a carburized steel component. If the Cr content is less than 0.90%, this effect cannot be achieved sufficiently even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Cr content is more than 2.00%, the hardness of steel material before forging increases, and the critical upsetting ratio decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the Cr content is 0.90 to 2.00%. The lower limit of the Cr content is preferably 0.95%, more preferably 1.00%, and further preferably 1.10%. The upper limit of the Cr content is preferably 1.95%, and more preferably 1.92%.

Mo: 0.10 to 0.40%

Molybdenum (Mo) improves the hardenability of steel material, and increases the strength of the core portion of a carburized steel component. If the Mo content is less than 0.10%, this effect cannot be achieved sufficiently even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Mo content is more than 0.40%, the hardness of steel material before forging increases, and the critical upsetting ratio decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the Mo content is 0.10 to 0.40%. The lower limit of the Mo content is preferably 0.11%, more preferably 0.12%, and further preferably 0.13%. The upper limit of the Mo content is preferably 0.38%, more preferably 0.36%, and further preferably 0.34%.

Al: 0.005 to 0.030%

Aluminum (Al) deoxidizes steel in the steel making process. Moreover, Al forms AlN when dissolved N is present in steel. However, in the steel material according to the present embodiment, N in steel is immobilized as TiN by the addition of Ti. For that reason, dissolved N is hardly present in the steel material. Consequently, Al does not form Al, and exists as dissolved Al in the steel material Al which exists in a dissolved state improves the machinability of the steel material. If the Al content is less than 0.005%, the above described effect cannot be achieved sufficiently even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Al content is more than 0.030%, the hardness of steel material before forging increases and the critical upsetting ratio decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the Al content is 0.005 to 0.030%. The lower limit of the Al content is preferably 0.010%, more preferably 0.011%, and further preferably 0.012%. The upper limit of the Al content is preferably 0.025%, more preferably 0.022%, and further preferably 0.020%.

Ti: 0.010 to less than 0.050%

Titanium (Ti) immobilizes N in steel material as TiN, and suppresses the formation of BN. As a result of this, Ti secures the amount of solute B and improves the hardenability of the steel material. Further, Ti forms TiC and thereby suppresses the coarsening of grains during carburizing treatment. If the Ti content is less than 0.010%, the above described effect cannot be achieved sufficiently even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Ti content is 0.050% or more, the precipitation amount of TiC excessively increases even if the contents of other elements are within the range of the present embodiment. In this case, the critical upsetting ratio of steel material before cold forging decreases. Therefore, the Ti content is 0.010 to less than 0.050%. The lower limit of the Ti content is preferably 0.012%, more preferably 0.014%, further preferably 0.016%, and further preferably 0.018%. The upper limit of the Ti content is preferably 0.048%, more preferably 0.046%, further preferably 0.044%, further preferably 0.042%, and further preferably 0.040%.

Nb: 0.010 to 0.030%

Niobium (Nb) combines with N and C in steel material to form Nb carbonitride. Nb carbonitride suppresses coarsening of grains by the pinning effect. If the Nb content is less than 0.010%, the above described effect cannot be achieved sufficiently even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Nb content is more than 0.030%, the effect will be saturated. Therefore, the Nb content is 0.010 to 0.030%. The lower limit of the Nb content is preferably 0.011%, more preferably 0.012%, further preferably 0.013%, and further preferably 0.014%. The upper limit of the Nb content is preferably 0.029%, more preferably 0.028%, further preferably 0.027%, further preferably 0.026%, and further preferably 0.025%.

N: 0.0080% or less

Nitrogen (N) is an unavoidably contained impurity. That is, the N content in steel material is more than 0%. N combines with B to form BN and reduces the amount of solute B. In this case, the hardenability of steel material deteriorates. If the N content is more than 0.0080%, it becomes unable to immobilize N in steel as TiN, making it difficult to ensure solute B which contributes to hardenability even if 0.010 to less than 0.050% of Ti is contained. Further, coarse TiN is formed. Coarse TiN acts as a starting point of cracking during forging and decreases the critical upsetting ratio of steel material before forging. Therefore, the N content is 0.0080% or less. The upper limit of the N content is preferably 0.0078%, more preferably 0.0076%, further preferably 0.0074%, and further preferably 0.0072%. The N content is preferably as low as possible. However, if the N content is decreased to the limit, the productivity deteriorates and the production cost increases. Therefore, in normal operation, the lower limit of the N content is preferably 0.0001%, more preferably 0.0010%, and further preferably 0.0020%.

O. 0.0030% or less

Oxygen (O) is an unavoidably contained impurity. That is, the O content in steel material is more than 0%. O forms oxides and deteriorates joining property when the steel material before carburizing treatment is welded. In this case, the fatigue strength of a carburized steel component decreases. Therefore, the O content is 0.0030% or less. The upper limit of the O content is preferably 0.0029%, more preferably 0.0028%, further preferably 0.0026%, further preferably 0.0024%, and further preferably 0.0022%. The O content is preferably as low as possible. However, if the O content is decreased to the limit, the productivity deteriorates and the production cost increases. Therefore, in normal operation, the lower limit the O content is preferably 0.0001%, more preferably 0.0005%, and further preferably 0.0010%.

B: 0.0003 to 0.0030%

Boron (B) improves the hardenability of steel material and increases the strength of a carburized steel component. If the B content is less than 0.0003%, the above described effect cannot be achieved sufficiently even if the contents of other elements are within the range of the present embodiment. On the other hand, if the B content is more than 0.0030%, the above described effect will be saturated. Therefore, the B content is 0.0003 to 0.0030%. The lower limit of the B content is preferably 0.0004%, more preferably 0.0005%, further preferably 0.0006%, and further preferably 0.0007%. The upper limit of the B content is preferably 0.0028%, more preferably 0.0026%, and further preferably 0.0024%.

Ca: 0.0005 to 0.0050%

Calcium (Ca) is contained in oxide and spheroidize the oxide. Spheroidized oxide is less likely to form a cluster. Further, Ca suppresses elongation of Mn sulfide. If the Ca content is less than 0.0005%, the above described effects cannot be achieved sufficiently even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Ca content is more than 0.0050%, coarse sulfide and coarse oxide are formed, thereby decreasing the fatigue strength of a carburized steel component. Therefore, the Ca content is 0.0005 to 0.0050%. The lower limit of the Ca content is preferably 0.0006%, more preferably 0.0007%, further preferably 0.0008%, further preferably 0.0009%, and further preferably 0.0010%. The upper limit of the Ca content is preferably 0.0048%, more preferably 0.0046%, further preferably 0.0040%, and further preferably 0.0035%.

Balance: Fe and impurities

The balance of the chemical composition of the steel material according to the present embodiment consists of Fe and impurities. Where, impurities mean components which are mixed in from ores and scrap as raw materials, or production environments when the steel material is industrially produced, and components which are not intentionally contained in the steel material.

[Optional elements]

The chemical composition of the steel material of the present embodiment may contain, in place of part of Fe, one or more kinds selected from the group consisting of Cu, Ni, and V. All of these elements increase the strength of a carburized steel component.

Cu: 0.50% or less

Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When Cu is contained, that is, the Cu content is more than 0%, Cu improves hardenability of steel material and increases the strength of a carburized steel component. Moreover, Cu is an element that does not form oxide or nitride in a gas atmosphere for gas carburization. For that reason, when Cu is contained, an oxide layer and a nitride layer of carburized layer surface, or an abnormal carburized layer caused thereby become less likely to be formed. If Cu is contained even in a small amount, the above described effect can be achieved to some extent. However, if the Cu content is more than 0.50%, the ductility of steel material in a high temperature range of 1000° C. or more deteriorates even if the contents of other elements are within the range of the present embodiment. In this case, the yield during continuous casting and rolling decreases. Further, the hardness of steel material before forging increases, and the critical upsetting ratio decreases. Therefore, the Cu content is 0.50% or less. That is, the Cu content is 0 to 0.50%. The lower limit of the Cu content is preferably more than 0%, more preferably 0.01%, further preferably 0.02%, and further preferably 0.05%. The upper limit of the Cu content is preferably 0.45%, more preferably 0.40%, and further preferably 0.35%.

Ni: 0.30% or less

Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When Ni is contained, that is, if the Ni content is more than 0%, Ni improves hardenability of steel material and increases the strength of a carburized steel component. If Ni is contained even in a small amount, the above described effect can be achieved to some extent. However, if the Ni content is more than 0.30%, the hardness of steel material before forging increases and the critical upsetting ratio decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the Ni content is 0.30% or less. That is, the Ni content is 0 to 0.30%. The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, and further preferably 0.05%. The upper limit of the Ni content is preferably 0.29%, more preferably 0.28%, and further preferably 0.25%.

V: 0.10% or less

Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When V is contained, that is, the V content is more than 0%. V forms carbide and increases the strength of the core portion of a carburized steel component. If V is contained even in a small amount, the above described effect can be achieved to some extent. However, if the V content is more than 0.10%, the cold forgeability of steel material deteriorates, and the critical upsetting ratio decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the V content is 0.10% or less. That is, the V content is 0 to 0.10%. The lower limit of the V content is preferably 0.01%, more preferably 0.02%, and further preferably 0.03%. The upper limit of the V content is preferably 0.09%, and more preferably 0.08%.

[Formulae (1) to (3)]

The chemical composition of the steel material of the present embodiment further satisfied the following Formulae (1) to (3):

0.140<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235  (1)

13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0   (2)

0.004<Ti—N×(48114)<0.030  (3)

where, each element symbol of the Formulae (1) to (3) is substituted by the content (mass %) of the corresponding element. If the corresponding element is an optional element, and not contained, the element symbol is substituted by “0”. Hereinafter, each formula will be described.

[Formula (1): hardness indicator]

Definition is made such that F1=C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al.

F1 is an indicator of the hardness of a carburized steel component which is produced using the steel material as the starting material.

In the steel material of the present embodiment, the C content is as low as 0.16% or less. Therefore, the structure of the steel material before forging has a significantly increased ferrite fraction as compared with a conventional steel material having a C content of about 0.20%. In this case, the hardness of the steel material is greatly affected not only by the C content (pearlite fraction) but also by the hardness of ferrite. F1 indicates the contribution of each alloying element to the solid solution strengthening of ferrite.

If F1 is 0.235 or more, the hardness of the steel material before cold forging is too high. In this case, the critical upsetting ratio of the steel material decreases. On the other hand, if F1 is 0.140 or less, the core portion hardness as a carburized steel component becomes deficient. Therefore, F1 is more than 0.140 to less than 0.235. F1 is preferably as low as possible within a range satisfying a hardenability indicator (F2) to be described later. The upper limit of F1 is preferably less than 0.230, more preferably 0.225, further preferably 0.220, further preferably 0.215, and further preferably 0.210. Note that an F1 value is a value obtained by rounding off the fourth decimal place of a calculated value.

[Formula (2): hardenability indicator]

Definition is made such that F2=(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1). F2 is an indicator regarding the hardenability of steel material.

As described above, B is effective in improving the hardenability of steel material and increasing the hardness of the core portion of a carburized steel component. On the other hand, when gas carburization (the gas-converter method) is performed as carburizing treatment, hardenability improving effect due to the inclusion of B is low in a carburized layer which is a near surface portion of the carburized steel component. This is because N in the atmosphere gas of the furnace invades into the near surface portion of a carburized steel component during carburizing treatment, and the solute B precipitates as BN so that the amount of solute B that contributes to the improvement of hardenability becomes deficient. Therefore, when gas carburizing treatment is performed, although B can increase the hardness of the core portion of a carburized steel component, it is difficult to contribute to increase of the hardness of the carburized layer of the carburized steel component. Therefore, to ensure hardenability in the carburized layer which is a near surface portion of the carburized steel component, it is necessary to utilize a hardenability improving element other than B.

F2 is constituted by elements other than B that contribute to the improvement of quenching. If F2 is 13.0 or less, it is not possible to sufficiently obtain an equal or larger carburized layer depth (a depth at which Vickers hardness is HV550 or more) compared to the above described conventional steel material (C content is about 0.20%) under the same carburizing treatment conditions. On the other hand, if F2 is 45.0 or more, the hardness of the steel material before cold forging increases and the critical upsetting ratio decreases. Therefore, F2 is more than 13.0 to less than 45.0. F2 is preferably as large as possible within a range satisfying the hardness indicator F1. The lower limit of F2 is preferably 13.2, more preferably 13.5, further preferably 14.0, further preferably 14.5, and further preferably 15.0. Note that an F2 value is a value obtained by rounding off the second decimal place of a calculated value.

[Formula (3): TiC precipitation amount indicator]

Definition is made such that F3=Ti—N×(48/14). F3 is an indicator regarding the TiC precipitation amount. When Ti is stoichiometrically excessive with respect to N, all N is fixed as TiN. That is, F3 means an excess amount of Ti other than the amount of Ti that has been consumed to form TiN. Where, “14” in F3 is the atomic weight of N, and “48” is the atomic weight of Ti.

Most of the excess amount of Ti defined by F3 combines with C to form TiC during the carburizing treatment. This TiC has a pinning effect of preventing coarsening of crystal grains during the carburizing treatment. If the content of each element in the chemical composition of the steel material satisfies the above described numerical range of the present embodiment, and F3 is 0.004 or less, the precipitation amount of TiC becomes deficient. In this case, it is not possible to suppress coarsening of crystal grains during carburizing treatment. As a result, the toughness of the carburized steel component is deteriorated, and the amount of deformation of the steel material after carburizing treatment is increased. On the other hand, if the content of each element in the chemical composition of the steel material satisfies the above described numerical range of the present embodiment, and F3 is 0.030 or more, the precipitation amount of TiC excessively increases, and the hardness of the steel material before cold forging increases and the critical upsetting ratio decreases. Therefore, F3 is more than 0.004 to less than 0.030. The lower limit of F3 is preferably 0.006, and more preferably 0.008. The upper limit of F3 is preferably 0.028, and more preferably 0.025. Note that an F3 value is a value obtained by rounding off the fourth decimal place of a calculated value.

A steel material whose chemical composition satisfies the hardness indicator F1, the hardenability indicator F2, and the TiC precipitation amount indicator F3 at the same time will have a larger critical upsetting ratio during cold forging than that of a conventional steel by being subjected to spheroidizing heat treatment. Then, after the carburizing treatment of this steel material, a carburized steel component having a hardened layer and core portion hardness of equal levels to those of a conventional steel can be obtained.

[Inclusions in steel material]

Further, in the steel material of the present embodiment, Mn sulfide and oxide in the steel satisfy the following conditions in a cross section parallel to the axial direction of the steel material (that is, the longitudinal direction of the steel material).

(I) The amount of Mn sulfide having, in mass %, a Mn content of 10.0% or more, a S content of 10.0% or more, and an O content of less than 10.0% is 70.0 pieces/mm² or less.

(II) The amount of oxide having, in mass %, an oxygen content of 10.0% or more is 25.0 pieces/mm² or less.

Here, Mn sulfide and oxide are defined as follows in the present description.

Mn sulfide: An inclusion having, in mass %, a Mn content of 10.0% or more, a S content of 10.0% or more, and an O content of less than 10.0% when the mass % of the inclusion is 100%.

Oxide: An inclusion having, in mass %, an O content of 10.0% or more when the mass % of the inclusion is 100%.

As described above, in the production process of a carburized steel component, when an intermediate member before carburizing treatment is integrally produced by joining a plurality of steel members by welding such as friction joining or laser joining, a HAZ region exists in the carburized steel component which is obtained by subjecting the intermediate member to carburizing treatment. The HAZ region may have a lower fatigue strength of weld-joint than other regions. In order to secure fatigue strength of weld-joint in the HAZ region, the amount of inclusions in the steel material is reduced as much as possible. If Mn sulfide and oxide satisfy the above described (I) and (II), it is possible to secure the fatigue strength of weld-joint of the HAZ region. As a result of that, it is possible to increase the fatigue strength of weld-joint of the carburized steel component integrated by joining.

[Measurement method of Mn sulfide and oxide]

The number of pieces of Mn sulfide and the number of pieces of oxide in steel can be measured by the following method. A sample is collected from the steel material. Specifically, as shown in FIG. 3, a sample is collected from an R/2 position (R is the radius of a steel bar) in the radial direction from the central axial line C1 of a steel material 1. The size of observation surface of the sample is L1×L2, where L1 is 10 mm and L2 is 5 mm. Further, the sample thickness L3 in the direction perpendicular to the observation surface is 5 mm. A normal line N of the observation surface is perpendicular to the central axial line C1 (that is, the observation surface is parallel to the axial direction of the steel material), and the R/2 position is a substantially central position of the observation surface.

The observation surface of the collected sample is mirror-polished, and 20 visual fields (evaluation area of 100 μm×100 m per one visual field) are randomly observed at a magnification of 1000 times using a scanning electron microscope (SEM).

Inclusions in each visual field are identified. For each of the identified inclusions, Mn sulfide and oxide are discriminated by using the energy dispersive X-ray spectroscopy (EDX). Specifically, by using the EDX, elemental analysis is performed at least two measurement points in each inclusion. Then, in each inclusion, an arithmetic mean value of the element content obtained at each measurement point is defined as the content (mass %) of each element in the inclusion. For example, when elemental analysis is performed at two measurement points in one inclusion, the arithmetic mean value of Mn content, the arithmetic mean value of S content, and the arithmetic mean value of O content obtained at the two measurement points are defined as Mn content (mass %), S content (mass %), and O content (mass %) in the inclusion.

In the elemental analysis results of the identified inclusions, an inclusion having, in mass %, a Mn content of 10.0% or more, a S content of 10.0% or more, and an O content of less than 10.0% is defined as Mn sulfide. Note that there is a case in which Ti and Ca are detected as elements other than Mn and S in the elemental analysis of inclusions. In this case as well, all of those that satisfy the above described condition are defined as Mn sulfide. Further, in the elemental analysis result of the discriminated inclusions, an inclusion having an O content of 10.0% or more is defined as oxide. There is a case in which Al, Si, Mg, Ca, Ti and the like are detected in an inclusion defined as oxide. In this case as well, if an inclusion satisfies the above described condition, it is discriminated as oxide. Of the inclusions, an inclusion containing, in mass %, 10.0% or more of S, 10.0% or more of Mn, and 10.0% or more of O is discriminated as oxide.

Inclusions to be subjected to the above described discrimination are inclusions having a circle-equivalent diameter of 0.5 μm or more. Where, the circle-equivalent diameter means the diameter of a circle when the area of each inclusion is converted into a circle having the same area.

If an inclusion has a circle-equivalent diameter of more than twice the beam diameter of EDX, the accuracy of elemental analysis will be improved. In the present embodiment, the beam diameter of the EDX used to identify inclusions is 0.2 μm. In this case, an inclusion having a circle-equivalent diameter of less than 0.5 μm cannot improve the accuracy of elemental analysis with EDX. An inclusion having a circle-equivalent diameter of less than 0.5 μm has a very small effect on fatigue strength. Therefore, in the present embodiment, Mn sulfide and oxide having a circle-equivalent diameter of 0.5 μm or more are to be measured. The upper limits of the circle-equivalent diameters of Mn sulfide and oxide are not particularly limited, but are, for example, 100 μm.

The total number of pieces per unit area (pieces/mm²) of Mn sulfide is determined based on the total number of pieces of Mn sulfide identified in each visual field and the total area of 20 visual fields. Further, the total number of pieces per unit area (pieces/mm²) of oxide is obtained based on the total number of pieces of oxide identified in each visual field and the total area of 20 visual fields.

In a steel material of the present embodiment, each element content in the chemical composition is within the above described range, and the hardness indicator F1 satisfies Formula (1), the hardenability indicator F2 satisfies Formula (2), and TiC precipitation amount indicator F3 satisfies Formula (3). Further, in a cross section parallel to the axial direction of the steel material, the amount of Mn sulfide having, in mass %, a Mn content of 10.0% or more, a S content of 10.0% or more, and an O content of less than 10.0% is 70.0 pieces/mm² or less, and the amount of oxide having, in mass %, an O content of 10.0% or more is 25.0 pieces/mm² or less. For that reason, even when welding is carried out before carburizing treatment, a carburized steel component after carburizing treatment has excellent fatigue strength.

[Microstructure of steel material]

The microstructure of a steel material of the present embodiment is not particularly limited. The steel material of the present embodiment may be an as-rolled member or may be spheroidized.

The radius in a cross section perpendicular to the axial direction (longitudinal direction) of the steel material of the present embodiment is defined as R (mm). The microstructure in the cross section perpendicular to the axial direction of the steel material is either one of the following (A) and (B).

-   -   (A) In the microstructure, the area fraction of bainite in at         least an outer layer region from the surface to a depth of 0.1R         is 95.0% or more.     -   (B) In the microstructure, at least the outer layer region from         the surface to a depth of 0.1R is composed of ferrite and         cementite, and the spheroidization ratio of cementite in the         outer layer region is 90.0% or more.

The microstructure of (A) described above is a microstructure when the steel material of the present embodiment is an as-rolled member. The microstructure of (B) described above is a microstructure when the steel material of the present embodiment is spheroidized.

FIG. 1 is a cross sectional view perpendicular to the longitudinal direction (axial direction) of the steel material of the present embodiment. In FIG. 1, the radius of the steel material 1 is defined as R (mm). At this moment, a region from the surface of the steel material 1 to a depth D of 0.1R is defined as an “outer layer region”. That is, the depth D (mm) means 10% of the radius R.

When the steel material of the present embodiment is an as-rolled member, at least the outer layer region has a bainite structure in the cross section perpendicular to the axial direction of the steel material, as described in (A). Where, the expression “has a bainite structure” as used herein means that the bainite area fraction is 95.0% or more. That is, in a cross section perpendicular to the axial direction of the steel material 1 of the present embodiment, at least the bainite area fraction of the outer layer region is 95.0% or more. Further, “at least the outer layer region has a bainite structure” means that the bainite region may be formed not only in the outer layer region but also in a region deeper than the outer layer region. Specifically, in FIG. 1, the depth of the bainite structure from the surface is at least 0.1R, and the depth of the bainite structure may be larger than 0.1R For example, the depth of the bainite structure may be 0.2R, 0.3R, or 1.0R. That is, the entire cross section perpendicular to the axial direction of the steel material 1 may have a bainite structure. When a steel material whose outer layer region has a bainite structure is subjected to spheroidizing treatment, cementite tends to be spheroidized. Therefore, the spheroidization ratio of cementite in the outer layer region will be 90.0% or more.

The microstructure of the steel material of the present embodiment may be (B) in place of (A). As described in the case where the steel material of the present embodiment is spheroidized (B), at least the outer layer region has a spheroidized cementite structure in the cross section perpendicular to the axial direction of the steel material of the present embodiment. Where, the “spheroidized cementite structure” means that the microstructure is composed of ferrite and cementite, and the spheroidization ratio of cementite in the microstructure is 90.0% or more. Further, “at least the outer layer region has a spheroidized cementite structure” means that the spheroidized cementite structure may be formed not only in the outer layer region but also in a region deeper than the outer layer region. Specifically, in FIG. 1, the depth of the spheroidized cementite structure from the surface may be at least 0.1R, and the depth of the spheroidized cementite structure may be deeper than 0.1R. For example, the depth of the spheroidized cementite structure may be 0.2R, 0.3R, or 1.0R. That is, the entire cross section perpendicular to the axial direction of the steel material 1 may have spheroidized cementite.

When the microstructure of the steel material of the present embodiment is (A), that is, when the steel material is an as-rolled member, the steel material is subjected to a spheroidizing heat treatment before cold forging. As a result, the microstructure of the steel material becomes (B).

When the microstructure of the steel material of the present embodiment is (B), it is possible to improve the cold forgeability (critical upsetting ratio) as compared with the structure in which the microstructure is composed of ferrite and pearlite.

Microstructure observation in the cross section perpendicular to the longitudinal direction of the steel material is performed by the following method. With reference to FIG. 2, in the cross section perpendicular to the longitudinal direction of the steel material 1, samples are collected from four locations (900 pitch in FIG. 2) at a position of d=0.05R, where the depth in the radial direction from the surface is d (mm). Further, samples are collected from four locations (90° pitch in FIG. 2) at a position of d=0.1R The surface of each collected sample is used as the observation surface. After being polished into a mirror surface, the observation surface of each sample is immersed in a nital etching solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed in a secondary electron image in three visual fields using the scanning electron microscope (SEM). The area of each visual field is 400 μm² (magnification of 5000 times). In each visual field, each phase of bainite, ferrite, pearlite, cementite, etc. can be distinguished as follows. When the observation surface is etched with the Nital etching solution, a phase having a lamellar structure can be identified as pearlite in SEM observation. A phase without a substructure within grain can be identified as ferrite. A phase in which a lath-like structure has developed from prior y grain boundaries can be identified as bainite. A granular, high-luminance phase can be identified as cementite. The luminance of laminar cementite in pearlite is about the same as the luminance of the above described granular cementite.

Photographic images of three visual fields are generated at each location (8 locations). The number of photographic images is 8 locations×3 visual fields=24. Each phase in each photographic image is identified by the method described above. Identification of phase by contrast can be realized by a well-known image processing method.

In the photographic image of each visual field, when the microstructure is mainly composed of bainite, it is judged that the microstructure of the steel material may be (A), and the bainite area fraction (%) is determined by the following method.

Bainite area fraction (%)=total area of bainite in 24 visual fields/total area of 24 visual fields×100

If the determined bainite area fraction is 95.0% or more, it is presumed that the outer layer region of at least 0.1R has bainite structure (that is, the microstructure is (A)). The calculation of the above described area fraction does not include precipitates other than cementite such as BN, TiC, TiN, and AlN, inclusions, and retained austenite.

In the photographic image of each visual field, when the microstructure is composed of ferrite and cementite, it is judged that the microstructure of the steel material may be (B), and the spheroidized cementite ratio (%) is determined by the following method.

The major axis (μm) and minor axis (μm) of each cementite are determined in each visual field (24 visual fields in total). Among the straight lines connecting any two points on the interface between cementite and matrix (ferrite), the length of the maximum straight line is defined as a major axis (μm) of the cementite. Among the straight lines connecting any two points on the interface between cementite and the matrix, the length of the straight line that intersects the major axis perpendicularly is defined as a minor axis (μm) of the cementite. Among determined cementite, those having a major axis of 0.1 μm or more are to be measured (counted). Next, an aspect ratio (major axis/minor axis) of each cementite to be measured is determined. The aspect ratio can be determined by well-known image processing. Cementite with an aspect ratio of 3.0 or less is defined as “spheroidized cementite”. The ratio (%) of the total number of pieces of spheroidized cementite in the 24 visual fields to the total number of pieces of cementite in the 24 visual fields is defined as a spheroidized cementite ratio (%). When the determined spheroidized cementite ratio is 90.0% or more, it is presumed that the outer layer region of at least 0.1R has a spheroidized cementite structure.

[Carburized steel component]

Next, description will be made on a carburized steel component which is produced by using the steel material of the present embodiment as the starting material.

A carburized steel component of the present embodiment includes a carburized layer formed in the outer layer and a core portion on the inner side of the carburized layer. The carburized layer has an effective hardened layer depth of 0.4 to less than 2.0 mm in thickness. Where, the effective hardened layer depth means a depth from the surface where Vickers hardness is HV 550 or more. In this carburized layer, Vickers hardness at a depth of 50 μm from the surface is preferably 650 to 1000 HV. Further, in the carburized layer, it is preferable that the microstructure at a depth of 0.4 mm from the surface preferably contains 90 to 100% of martensite in area %, and Vickers hardness is 600 to 900 HV.

When Vickers hardness in the carburized layer at a depth of 50 m from the surface is 650 to 1000 HV, the wear resistance and fatigue strength are further increased. More preferably, Vickers hardness at a depth of 50 μm from the surface is 700 to 1000 HV.

When the microstructure in the carburized layer at a depth of 0.4 mm from the surface contains 90 to 100% of martensite, and Vickers hardness in the carburized layer at a depth of 0.4 mm from the surface is 600 to 900 HV, the surface fatigue strength and the fatigue strength are further increased. More preferably, Vickers hardness at a depth of 0.4 mm from the surface is 620 to 900 HV.

Moreover, in the core portion at a depth of 2.0 mm from the surface, Vickers hardness is preferably 250 to 500 HV. Further, the chemical composition at a depth of 2.0 mm from the surface is the above described chemical composition. More preferably, Vickers hardness at a depth of 2.0 mm from the surface is 270 to 450 HV. If the microstructure at a depth of 2.0 mm from the surface contains at least one of martensite and bainite, the above described effect can be further achieved, which is preferable.

The microstructure at a depth of 0.4 mm from the surface of the carburized steel component is determined by the following method. A sample which includes, on the surface thereof, a position of 0.4 mm depth from the surface of the carburized steel component is collected. The surface of the sample is etched with a Picral solution. In the surface after etching, any three visual fields are observed in a secondary electron image in any three visual fields by using the SEM. The area of each visual field is 400 μm² (magnification of 5000 times). In the SEM observation, martensite and bainite (including tempered martensite and tempered bainite), ferrite, pearlite, and cementite can be distinguished as follows. Specifically, a phase having a lamellar structure can be identified as pearlite. A phase without a substructure in the grain can be identified as ferrite. A phase including a lath-like structure can be identified as martensite and bainite. Note that tempered martensite and tempered bainite contain a lath-like structure, and further include carbide in the lath. A granular, high-luminance phase can be identified as cementite. The luminance of the laminar cementite in pearlite is similar to the luminance of the granular cementite described above. As described above, martensite and bainite both contain a lath-like structure, and in the present description, martensite and bainite are not distinguished in the microstructure of carburized steel component.

The total area of martensite in the three visual fields of the sample at a position of 0.4 mm depth is determined. The ratio of the determined total area of martensite to the total area of the three visual fields is defined as the area fraction (%) of martensite at a position of 0.4 μm depth. In the calculation of the area fraction of martensite: ferrite, pearlite, martensite and bainite, tempered martensite, tempered bainite, spheroidized cementite, and cementite are taken into consideration. The above described calculation of the area fraction does not include precipitates other than cementite such as BN, TiC, TiN, and AlN, inclusions, retained austenite, etc.

Vickers hardness of a carburized steel component is measured by the following method. A cross section perpendicular to any surface of the carburized steel component is the measurement surface. On the measurement surface, Vickers hardness at a position of 50 μm depth from the surface and Vickers hardness at a position of 0.4 mm depth from the surface are determined by Vickers hardness test in conformity with JIS Z 2244 (2009) using a micro Vickers hardness tester. The test force is 0.49N. Vickers hardness HV is measured in 10 locations at a position of 50 μm depth, and an average value thereof is assumed as Vickers hardness HV at a position of 50 μm depth. Further, Vickers hardness HV is measured in 10 locations at a position of 0.4 mm depth, and an average value thereof is assumed as Vickers hardness HV at a position of 0.4 mm depth. If Vickers hardness at a position of 0.4 mm depth is 550 HV or more, it is judged that the carburized layer depth is at least 0.4 mm or more. Further, in the measurement surface, Vickers hardness at a position of 2.0 mm depth from the surface is determined by a Vickers hardness test in conformity with JIS Z 2244 (2009) using a micro Vickers hardness tester. The load at the time of the test is 0.49N. Vickers hardness HV is measured in 10 locations at a position of 2.0 mm depth, and an average value thereof is assumed as Vickers hardness HV at a position of 2.0 mm depth. The measurement surface of Vickers hardness is not particularly limited, but may be a cut cross section orthogonal to the axial direction (longitudinal direction) of the carburized steel component.

[Production methods of steel material and carburized steel component]

Production methods of a steel material and a carburized steel component according to the present embodiment will be described.

[Production method of steel material]

First, one example of the production method of a steel material according to the present embodiment will be described. The one example of the production method of a steel material includes a steelmaking process, a casting process, a hot working process, and a cooling process. Hereinafter, each process will be described.

[Steelmaking process]

A steelmaking process includes a smelting process and a casting process.

[Smelting process]

In the smelting process, first, molten pig iron produced by a well-known method is subjected to smelting (primary smelting) in a converter. The molten steel discharged from the converter is subjected to secondary smelting. In the secondary smelting, alloying elements are added to the molten steel to produce a molten steel satisfying the above described chemical composition.

Specifically, Al is added to the molten steel discharged from the converter to carry out deoxidizing treatment. After the deoxidizing treatment, the slag removal treatment is performed. After the slag removal treatment, secondary smelting is performed. In the secondary smelting, for example, complex smelting is performed. For example, first, a smelting treatment using LF (Ladle Furnace) or VAD (Vacuum Arc Degassing) is performed. Further, RH (Ruhrstahl-Hausen) vacuum degassing treatment is performed. Thereafter, final adjustment of other alloy components excluding Si and Ca is performed.

After performing the secondary smelting and adjusting the components of the molten steel other than Si and Ca, the following treatments (a heating and holding process and a final component adjusting process) are carried out on the molten steel.

[Heating and holding process]

The molten steel in the ladle after the secondary smelting (final component adjustment) is heated at a temperature of 1500 to 1600° C. for a holding time ts which is at least twice the uniform mixing time t(s) calculated by the following formula.

τ=800×ε^(−0.4)

ε=((6.18×V_(g)×T₁)/M₁)ln(1+(h₀/(1.46×10⁻⁵×P₀)))

Where, V_(g): gas flow rate (Nm³/min). M₁: molten steel mass in ladle (ton), T₁: molten steel temperature (K), h₀: gas blowing depth (m), P₀: molten steel surface pressure (Pa), ε: stirring power value (W/ton), τ: uniform mixing time (s).

If the holding time ts is less than twice the uniform mixing time τ, oxides present in the molten steel in the ladle cannot be sufficiently aggregated and coalesced. Therefore, floating removal of the oxides cannot be performed, and the number of pieces of oxide will increase. When the holding time ts is less than twice the uniform mixing time τ. Mg and the like, which are mixed in from the slag, combine with S in the molten steel to form MgS and the like, resulting in a state in which MgS is dispersed in the molten steel. This dispersed MgS becomes a precipitation site of MnS. As a result, the number of pieces of Mn sulfide will increase.

When the holding time ts is twice or more of the uniform mixing time τ, the number of pieces of oxide in the steel can be suppressed. Further, since MgS once formed becomes MgO by reoxidation, the number of precipitation sites of MnS is decreased, and as a result, the number of pieces of Mn sulfide in the steel can be suppressed as well. As a result, after the final component adjusting process, which is the following step, the amount of Mn sulfide will be 70.0 pieces/mm² or less, and the amount of oxide will be 25.0 pieces/mm² or less.

[Final component adjusting process]

Si and Ca are added to the molten steel after the heating and holding process to produce molten steel satisfying the above described chemical composition and Formulae (1) to (3). Si and Ca may be added to molten steel as individual raw materials. A Si—Ca alloy may be added to molten steel as a raw material.

When Si and Ca are added to the molten steel that has been sufficiently uniformly heated in the heating and holding process, oxide is modified from Al₂O₃ into a compound inclusion containing SiO₂ and CaO, and Mn sulfide is also modified into sulfide containing Ca. Therefore, on the precondition that the holding time ts is twice or more of the uniform mixing time r, the amount of Mn sulfide will be 70.0 pieces/mm² or less, and the amount of oxide will be 25.0 pieces/mm² or less.

If Si is added to the molten steel before Al is added, deoxidation is not sufficiently performed, and as a result, the amount of oxide will be more than 25.0/mm². By adding Si and Ca to the molten steel after Al is added, the amount of Mn sulfide will be 70.0 pieces/mm² or less, and the amount of oxide will be 25.0 pieces/mm² or less. Therefore, in the present embodiment, Al is added to the molten steel, and thereafter Si and Ca are added. The order of addition of Si and Ca is not particularly limited. Si and Ca may be added at the same time. Either of Si and Ca may be added first.

[Casting process]

A starting material (cast piece or ingot) is produced by using the molten steel produced by the above described smelting process. Specifically, the cast piece is produced by a continuous casting method using the molten steel. Alternatively, the molten steel may be used to obtain an ingot by an ingot-making process. Using this cast piece or ingot, a hot working process which is the next step is carried out. Hereinafter, the cast piece or ingot is referred to as a “starting material”.

[Hot working process]

In the hot working process, the starting material (bloom or ingot) prepared in the casting process is subjected to hot working to produce a steel material. The shape of the steel material is not particularly limited, but is, for example, a steel bar or wire rod. In the following description, as an example, a case where the steel material is a steel bar will be described. However, even if the steel material has a shape other than the steel bar, it can be produced by the same hot working process.

The hot working includes a rough rolling process and a finish rolling process. In the rough rolling process, the starting material is subjected to hot working to produce a billet. For the rough rolling process, for example, a blooming mill is used. The starting material is subjected to blooming by the blooming mill to produce a billet. When a continuous rolling mill is installed in the downstream of the blooming mill, the billet after the blooming may be further subjected to hot rolling by using the continuous rolling mill to produce a billet of a smaller size. In the continuous rolling mill, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. Through the above described process, the starting material is produced into a billet in the rough rolling process. The heating temperature in the rough rolling process is not particularly limited, but is, for example, 1100 to 1300° C.

In the finish rolling process, the billet is heated by using a heating furnace or a soaking pit. The billet after heating is subjected to hot rolling by using a continuous rolling mill to produce a steel material (steel bar). The heating temperature in the finish rolling process is not particularly limited, but is, for example, 1000 to 1250° C.

[Cooling process]

In the cooling process, the steel material immediately after completion of hot working process is cooled. Specifically, the steel material is cooled at a cooling rate of more than 1.0° C./sec to 30.0° C./sec in a temperature range in which the surface temperature of the steel material is 800° C. to 500° C.

In the steel material produced by the above described production process, the area fraction of bainite at least in the outer layer region from the surface to 0.1R depth is 95.0% or more in the microstructure in a cross section perpendicular to the axial direction. That is, by the above described production process, a steel material (as-rolled member) whose microstructure is (A) is produced.

[Spheroidizing heat treatment process]

The steel material after the cooling process may be further subjected to a spheroidizing heat treatment process to obtain the steel material of the present embodiment as a steel material having the microstructure (B). That is, in this case, the spheroidizing heat treatment is performed to produce the steel material having the microstructure (B).

The spheroidizing heat treatment may be a well-known method. The spheroidizing heat treatment is performed by, for example, the following method. The steel material after the above described cooling process is heated to a temperature directly below or directly above A_(c1) point (the temperature at which austenite begins to be formed during heating) (for example, A_(c1) point+50° C. or less), held for a predetermined time, and thereafter slowly cooled. Alternatively, processing to heat the steel material after the cooling process to a temperature just above the A_(c1) point and to cool it to a temperature just below A_(r1) point (the temperature at which austenite completes the transformation to ferrite, or ferrite and cementite during cooling) may be repeatedly performed several times. Alternatively, the steel material after the cooling process may be quenched once, and thereafter tempered in a temperature range of 600 to 700° C. for 3 to 100 hours. Note that the method of spheroidizing heat treatment may adopt well-known annealing or spheroidizing heat treatment method as described above, and is not particularly limited.

In the steel material produced by performing the spheroidizing heat treatment, in the microstructure in a cross section perpendicular to the axial direction, at least the outer layer region from the surface to 0.1R depth is composed of ferrite and cementite, in which spheroidization ratio of the cementite will be 90.0% or more. That is, the steel material whose microstructure is (B) is produced by the above described production process.

Through the production process described so far, the steel material of the present embodiment can be produced.

[Production method of carburized steel component]

Next, one example of the production method of a carburized steel component which uses the steel material of the present embodiment as the starting material will be described. The present production method includes: a cold working process in which cold forging is performed on the above described steel material to produce a plurality of intermediate members; a welding process in which, as needed, the produced plurality of intermediate members are welded into an integrated product; a cutting work process in which, as needed, cutting work is performed on the intermediate members; a carburizing treatment process in which carburizing treatment is performed on the intermediate members; and a tempering process in which tempering is performed on the intermediate steel material after carburizing treatment process. Note that in the present description, the carburizing treatment includes carbonitriding treatment.

When the microstructure of the steel material of the present embodiment is (A), that is, when the steel material of the present embodiment is an as-rolled member, the steel material is subjected to the above described spheroidizing heat treatment process, and thereafter to cold forging process. Before carrying out the spheroidizing heat treatment process on the steel material having the microstructure (A), a cold drawing process such as a wire drawing process is carried out as needed.

[Cold forging process]

In the cold forging process, cold forging is performed on the steel material produced by the above described production method to shape it and produce a plurality of intermediate members. The cold forging conditions such as the working ratio and the strain rate in this cold forging process are not particularly limited. As the cold forging conditions, suitable conditions may be appropriately selected. The plurality of intermediate members are welded and integrated in the welding process of the next step.

[Welding process]

The welding process is an optional process and does not need to be carried out. When carried out, in the welding process, the above described plurality of intermediate members are welded and integrated by friction joining or laser joining. The welding method is not particularly limited. After welding, the joining surface of the intermediate member may be formed to be flat by machining. In the steel material of the present embodiment, the amount of Mn sulfide is 70.0 pieces/mm² or less and the amount of oxide is 25.0 pieces/mm² or less. Therefore, the steel material of the present embodiment has excellent joining property, and even when the intermediate members are welded to form a carburized steel component, the fatigue strength of weld-joint of the carburized steel component is excellent.

[Cutting work process]

The cutting work process is an optional process and does not need to be carried out. When carried out, in the cutting work process, the intermediate member after the cold forging process and before the carburizing treatment process described later is subjected to the cutting work to impart a shape thereto. By performing the cutting work, it is possible to impart a precise shape to the carburized steel component, which is difficult to be done only by the cold forging process.

[Carburizing treatment process]

In the carburizing treatment process, carburizing treatment is performed on the intermediate member (integrally joined intermediate member when the welding process is performed). In the carburizing treatment process, a well-known carburizing process is performed. The carburizing treatment process includes a carburizing process, a diffusion process, and a quenching process.

The carburizing treatment conditions in the carburizing process and the diffusion process may be appropriately adjusted. The carburizing temperature in the carburizing process and the diffusion process is, for example, 830 to 1100° C. The carbon potential in the carburizing process and the diffusion process is, for example, 0.5 to 1.2%. The holding time in the carburizing process is, for example, 60 minutes or more, and the holding time in the diffusion process is 30 minutes or more. It is preferable that the carbon potential in the diffusion process is lower than the carbon potential in the carburizing process. However, the conditions in the carburizing process and the diffusion process are not limited to the above-mentioned conditions.

After the diffusion process, a well-known quenching process is performed. In the quenching process, the intermediate member after the diffusion process is held at a quenching temperature equal to or higher than the A_(r3) transformation point. The holding time at the quenching temperature is not particularly limited, but is, for example, 30 to 60 minutes. Preferably, the quenching temperature is lower than the carburizing temperature. The temperature of the quenching medium is preferably room temperature to 250° C. The quenching medium is, for example, water or oil. Further, as needed, subzero treatment may be performed after quenching.

[Tempering process]

A well-known tempering process is performed on the intermediate member after the carburizing treatment process. The tempering temperature is, for example, 100 to 250° C. The holding time at the tempering temperature is, for example, 60 to 150 minutes.

[Other processes]

As needed, the carburized steel component after the tempering process may be further subjected to grinding work or shot peening treatment. By performing the grinding work, a precise shape can be imparted to the carburized steel component. Further, by performing shot peening treatment, compressive residual stress is introduced into the near surface portion of the carburized steel component. Compressive residual stress suppresses the generation and growth of a fatigue crack. Therefore, the fatigue strength of carburized steel component is increased. For example, when the carburized steel component is a gear, the fatigue strength of the tooth root and the tooth surface of the carburized steel component can be increased. The shot peening treatment may be performed by a well-known method. For example, the shot peening treatment is preferably performed by using shot grains having a diameter of 0.7 mm or less, under a condition of an arc height of 0.4 mm or more.

As described above, the steel material of the present embodiment can be applied as a starting material for a carburized steel component which is integrated by welding a plurality of intermediate members. Of course, the steel material of the present embodiment can be further applied as a starting material for carburized steel components without performing welding.

EXAMPLE

The effect of one aspect of the steel material of the present disclosure will be described more specifically by way of an example. The condition in the example is one condition example adopted to confirm the feasibility and effect of the steel material of the present disclosure. The steel material of the present disclosure is not limited to this one condition example. The steel material of the present disclosure may adopt various conditions as long as the object of the present disclosure is achieved without deviating from the gist of the present disclosure.

Molten steels having the chemical compositions shown in Table 1 were prepared. At this moment, smelting was performed under the conditions shown in Table 2. The molten steels after smelting were cast by continuous casting to obtain cast pieces. A blank portion in Table 1 means that the content of the corresponding element was below a detection limit. That is, the blank portion means that the corresponding element content was below the detection limit in the least significant digit thereof. For example, in the case of the Cu content in Table 1, the least significant digit is the second decimal place. Therefore, the Cu content of Test Number 1 means that it was not detected in the digit up to the second decimal place (significant figures were 0% in the content up to the second decimal place).

TABLE 1 Chemical composition (in mass %, with Steel the balance being Fe and impurities) Number C Si Mn P S Cr Mo Al Ti Nb N A 0.10 0.08 0.52 0.009 0.021 1.39 0.28 0.018 0.034 0.018 0.0075 B1 0.12 0.06 0.49 0.011 0.014 1.47 0.22 0.013 0.029 0.012 0.0042 C1 0.15 0.11 0.45 0.008 0.019 1.57 0.24 0.012 0.024 0.019 0.0052 D 0.12 0.03 0.59 0.007 0.017 0.99 0.33 0.018 0.029 0.026 0.0050 E 0.14 0.15 0.40 0.009 0.024 1.16 0.24 0.014 0.033 0.021 0.0064 F 0.12 0.13 0.48 0.006 0.017 1.84 0.16 0.017 0.028 0.018 0.0043 G 0.09 0.16 0.57 0.010 0.013 1.63 0.25 0.014 0.027 0.021 0.0049 H 0.14 0.07 0.51 0.009 0.018 0.91 0.39 0.021 0.041 0.024 0.0079 I 0.09 0.12 0.59 0.012 0.005 1.96 0.22 0.024 0.028 0.022 0.0056 J 0.10 0.02 0.42 0.009 0.008 1.89 0.13 0.021 0.048 0.019 0.0079 K 0.12 0.15 0.43 0.009 0.013 1.72 0.26 0.026 0.036 0.021 0.0043 L 0.17 0.06 0.41 0.010 0.021 1.38 0.27 0.021 0.031 0.018 0.0058 M 0.07 0.07 0.58 0.011 0.008 1.69 0.22 0.024 0.029 0.028 0.0041 N 0.12 0.11 0.54 0.011 0.010 1.48 0.31 0.018 0.024 0.016 0.0041 O 0.12 0.12 0.53 0.009 0.011 1.52 0.28 0.021 0.025 0.017 0.0117 P 0.14 0.17 0.54 0.010 0.008 1.81 0.22 0.018 0.032 0.022 0.0068 Q 0.09 0.01 0.43 0.011 0.022 1.03 0.12 0.014 0.025 0.015 0.0050 R 0.09 0.18 0.49 0.008 0.008 1.88 0.35 0.022 0.025 0.020 0.0049 S 0.13 0.03 0.41 0.012 0.012 0.91 0.11 0.019 0.037 0.012 0.0056 T 0.11 0.11 0.48 0.012 0.009 1.48 0.21 0.028 0.049 0.012 0.0032 U 0.14 0.06 0.44 0.008 0.008 1.52 0.23 0.016 0.016 0.022 0.0065 V 0.12 0.06 0.49 0.008 0.016 1.18 0.23 0.022 0.026 0.013 0.0057 W 0.16 0.24 0.51 0.011 0.018 1.26 0.21 0.024 0.033 0.016 0.0072 X 0.10 0.31 0.49 0.009 0.011 1.92 0.37 0.021 0.026 0.019 0.0042 B2 0.11 0.07 0.51 0.010 0.015 1.49 0.21 0.013 0.028 0.013 0.0044 C2 0.14 0.12 0.47 0.009 0.017 1.56 0.22 0.014 0.025 0.018 0.0051 B3 0.12 0.07 0.48 0.012 0.014 1.48 0.23 0.013 0.031 0.011 0.0043 C3 0.15 0.11 0.44 0.008 0.018 1.57 0.23 0.012 0.025 0.020 0.0053 Chemical composition (in mass %, with Steel the balance being Fe and impurities) Number O B Ca Cu Ni V F1 F2 F3 A 0.0016 0.0022 0.0010 0.177 28.8 0.008 B1 0.0022 0.0023 0.0013 0.189 25.6 0.015 C1 0.0014 0.0021 0.0012 0.228 27.2 0.006 D 0.0019 0.0024 0.0007 0.188 25.9 0.012 E 0.0028 0.0027 0.0031 0.218 20.5 0.011 F 0.0015 0.0022 0.0006 0.205 28.1 0.013 G 0.0016 0.0024 0.0011 0.187 34.9 0.010 H 0.0027 0.0025 0.0015 0.212 24.6 0.014 I 0.0014 0.0023 0.0009 0.19 0.22 0.217 41.4 0.009 J 0.0023 0.0027 0.0044 0.18 0.177 22.8 0.021 K 0.0022 0.0019 0.0013 0.15 0.218 31.7 0.021 L 0.0019 0.0018 0.0021 0.235 23.5 0.011 M 0.0021 0.0016 0.0012 0.151 32.5 0.015 N 0.0042 0.0019 0.0011 0.206 33.2 0.010 O 0.0028 0.0021 0.0014 0.207 32.1 −0.015 P 0.0014 0.0023 0.0019 0.18 0.251 37.0 0.009 Q 0.0016 0.0016 0.0012 0.137 14.3 0.008 R 0.0012 0.0019 0.0007 0.21 0.29 0.232 45.8 0.008 S 0.0027 0.0025 0.0018 0.179 12.6 0.018 T 0.0021 0.0022 0.0012 0.189 25.8 0.038 U 0.0024 0.0018 0.0009 0.207 24.8 −0.006 V 0.0019 0.0023 0.0008 0.06 0.187 22.2 0.006 W 0.0021 0.0017 0.0021 0.264 25.9 0.008 X 0.0015 0.0018 0.0011 0.229 46.9 0.012 B2 0.0021 0.0024 0.0011 0.183 26.3 0.013 C2 0.0016 0.0022 0.0013 0.221 27.1 0.008 B3 0.0025 0.0022 0.0011 0.191 26.0 0.016 C3 0.0015 0.0022 0.0011 0.227 26.3 0.007

TABLE 2 Steel material for carburized steel component Average cooling rate Critical (° C./sec) at 800 Micro- Micro- Number Number compression ratio to 500° C. in structure structure of pieces of pieces As-rolled SA Test Steel Steelmaking Steelmaking cooling of as-rolled of SA of MnS of oxide member member Number Number condition (1) condition (2) process member member (pieces/mm²) (pieces/mm²) (%) (%) 1 A 4.0 1 2.2 Y Y 43.0 21.0 60 80 2 B1 3.0 1 1.2 Y Y 51.0 19.0 65 78 3 C1 4.0 1 3.0 Y Y 42.0 10.0 60 80 4 D 4.0 1 10.0 Y Y 46.0 15.0 55 81 5 E 3.0 1 18.0 Y Y 59.0 17.0 65 80 6 F 3.0 1 18.0 Y Y 41.0 18.0 50 80 7 G 5.0 1 2.5 Y Y 52.0 19.0 60 78 8 H 3.0 1 8.0 Y Y 50.0 13.0 60 80 9 I 2.5 1 1.5 Y Y 38.0 15.0 64 78 10 J 3.0 1 9.5 Y Y 41.0 10.0 60 77 11 K 4.0 1 3.5 Y Y 34.0 22.0 58 80 12 L 4.0 1 12.0 Y Y 29.0 12.0 48 74 13 M 3.0 1 3.8 Y Y 38.0 14.0 60 79 14 N 4.0 1 5.1 Y Y 51.0 29.0 54 80 15 O 3.0 1 2.0 Y Y 44.0 18.0 49 73 16 P 5.0 1 2.5 Y Y 21.0 19.0 49 74 17 Q 4.0 1 5.0 Y Y 39.0 20.0 65 79 18 R 3.0 1 12.0 Y Y 63.0 11.0 40 71 19 S 2.5 1 8.0 Y Y 59.0 17.0 68 78 20 T 3.0 1 6.0 Y Y 43.0 16.0 48 74 21 U 4.0 1 6.0 Y Y 39.0 22.0 65 78 22 B2 1.5 1 4.6 Y Y 87.0 45.0 64 79 23 C2 1.0 1 13 Y Y 76.0 37.0 56 78 24 B3 4.0 2 7.0 Y Y 64.0 42.0 57 80 25 C3 3.0 2 13.0 Y Y 65.0 45.0 53 81 26 B1 3.0 1 35.0 N Y 49.0 21.0 30 78 27 C1 4.0 1 32.0 N Y 41.0 11.0 34 79 28 V 4.0 1 14.0 Y Y 42.0 19.0 56 77 29 W 3.0 1 6.0 Y Y 51.0 12.0 42 70 30 X 4.0 1 9.0 Y Y 44.0 16.0 46 72 1: Al addition → Si and Ca addition 2: Si addition → Al and Ca addition

“Steelmaking condition (1)” in Table 2 indicates a ratio (=ts/τ) of holding time ts at 1500 to 1600° C. after secondary smelting to uniform mixing time τ. “Steelmaking condition (2)” in Table 2 indicates the order of addition of Al, Si, and Ca. In the “steelmaking condition (2)” column, “1” means that Al was added to perform deoxidization, and thereafter Si and Ca were added. The numeral “2” means that Al and Ca were added after Si was added. Note that for Test Numbers 22 and 24, the steelmaking process was carried out with the chemical composition of Steel Number B1 as the target. For Test Numbers 23 and 25, the steelmaking process was carried out with the chemical composition of Steel Number C1 as the target.

The produced cast piece was heated at 1100 to 1300° C. and thereafter subjected to a rough rolling process to obtain a billet having a cross section of 162 mm×162 mm, which was perpendicular to the longitudinal direction. A finish rolling process was carried out using this billet. In the finish rolling process, hot rolling was performed by a continuous rolling mill using billets heated to 1000 to 1250° C. to produce a steel bar which had a circular cut cross section orthogonal to the longitudinal direction and in which the diameter of the cut cross section was 30 mm. A cooling process was carried out on the steel bars immediately after the finish rolling process. The average cooling rate (° C./sec) at 800 to 500° C. in the cooling process was as shown in Table 2. For each test number, a plurality of steel bars (hereinafter referred to “as-rolled members”) after the cooling process were prepared.

In each test number, a spheroidizing heat treatment process (SA process: Spheroidizing Annealing) was carried out on some of the prepared plurality of steel bars. In the spheroidizing heat treatment, the above described steel bars were heated to 740° C. Thereafter, the steel bars were slowly cooled at a cooling rate of 8° C./hr until the temperature thereof reached 650° C. A steel bar, which was subjected to a spheroidizing heat treatment process by air-cooling from a steel material temperature of 650° C. to room temperature, was produced (hereinafter referred to as an “SA member”). By the above described production method, steel materials (as-rolled members, and SA members) of each test number were produced.

[Evaluation test]

The following tests were conducted on the steel materials of each test number.

[Microstructure Observation Test]

Microstructure observation was performed by the following method. Specifically, in the cross section perpendicular to the longitudinal direction of the steel material of each test number, samples were collected from four locations at d=0.05R position (90° pitch in FIG. 2), where the depth in the radial direction from the surface is d (mm). Further, samples were collected from four locations (90° pitch in FIG. 2) at d=0.1R position. A surface S of each collected sample was used as the observation surface. After being polished into a mirror surface, the observation surface of each sample was immersed in a Nital etching solution for about 10 seconds to reveal the structure by etching. The etched observation surface was observed with a secondary electron image in three visual fields using the SEM. The area of each visual field was 400 μm² (magnification of 5000 times). In each visual field, bainite, and other phases (ferrite, pearlite, cementite, etc.) were able to be distinguished as described above.

As for the as-rolled members, bainite area fraction (%) was determined by the following method.

Bainite area fraction (%)=total area of bainite in 24 visual fields/total area of 24 visual fields×100

If the determined bainite area fraction was 95.0% or more, it was presumed that the outer layer region of at least 0.1R depth from the surface had a bainite structure (“Y” in “Microstructure of as-rolled member” in Table 2). On the other hand, when the determined bainite area fraction is less than 95.0%, it was presumed that the outer layer region at 0.1R depth from the surface had not a bainite structure (“N” in “Microstructure of as-rolled member” in Table 2).

As for the SA members, a spheroidized cementite ratio (%) was determined by the following method. First, a major axis (μm) and a minor axis (μm) of each cementite were determined in each visual field (24 visual fields). Among the straight lines connecting any two points on the interface between cementite and matrix (ferrite), a maximum length of the straight lines was defined as a major axis (μm) of the cementite. Among the straight lines connecting any two points on the interface between cementite and matrix, the length of the straight line that intersects the major axis perpendicularly was defined as a minor axis (μm) of the cementite. Among determined cementite, those having a major axis of 0.1 μm or more were measured (counted). Next, an aspect ratio (major axis/minor axis) of each cementite to be measured was determined. Cementite with an aspect ratio of 3.0 or less was defined as “spheroidized cementite”. A ratio (%) of the total number of pieces of spheroidized cementite in 24 visual fields to the total number of pieces of cementite in 24 visual fields was defined as a spheroidized cementite ratio (%). When the determined spheroidized cementite ratio was 90.0% or more, it was presumed that at least the outer layer region of 0.1R had a spheroidized cementite structure (“Y” in “Microstructure of SA member” in Table 2). On the other hand, when the determined area fraction of spheroidized cementite was less than 90.0%, it was presumed that the outer layer region at 0.1R depth from the surface had no spheroidized cementite structure (“N” in “Microstructure of SA member” in Table 2). Note that any of microstructures of 24 visual fields of SA members of each test number was a structure composed of ferrite and cementite. That is, in the SA member of each test number, all of the microstructure of the outer layer region was a structure composed of ferrite and cementite.

[Measurement test of critical compression ratio]

A compression test specimen was prepared from the steel material of each test number, which has a diameter of 30 mm, such that the longitudinal direction of the steel material was the compression direction. The diameter of the compression test specimen was 29.5 mm and the length was 44 mm. The central axis of the compression test specimen substantially coincided with the central axis of the steel material. A notch was formed in the circumferential direction at the central position in the longitudinal direction of the compression test specimen. The notch angle was 30°, the notch depth was 0.8 mm, and the radius of curvature of a notch tip was 0.15 mm. The compression test specimens were collected from the as-rolled members and the SA members, respectively. Hereinafter, among the test specimens, one collected from the as-rolled member is referred to as “as-rolled test specimen”, and one collected from the SA member is referred to as “SA test specimen”.

The critical compression test was conducted on the above described compression test specimens (as-rolled test specimen, and SA test specimen) by the following method. A 500 ton hydraulic press was used for the critical compression test. Each test specimen was subjected to cold compression using an arresting die at a speed of 10 mm/min. Compression was stopped when a microcrack of 0.5 mm or more occurred in the vicinity of the notch, and the compression ratio (%) at that time was calculated. This measurement was performed 10 times in total to determine a compression ratio (%) at which the cumulative failure probability was 50%, and this compression ratio was defined as the critical compression ratio (%). Table 2 shows the critical compression ratio (%) of each test number.

As described above, the as-rolled member may be subjected to cold drawing work such as wire drawing before the spheroidizing heat treatment process. In this case, the as-rolled member needs to have workability that prevents breakage due to internal cracks (Chevron cracks) in cold drawing work. Therefore, as regards the as-rolled test specimen, it was judged that critical upsetting ratio was excellent when the critical compression ratio was 50% or more. Note that for test numbers in which the critical compression ratio of as-rolled test specimen was less than 50%, the subsequent evaluation tests of carburized steel component was not conducted.

As regards the SA test specimen, since the critical compression ratio of the conventional steel material used as the starting material for carburized steel components was about 65%, so if the critical compression ratio was 75% or more, which could be regarded as a value clearly higher than the aforementioned value, the critical upsetting ratio was judged to be excellent. Note that for the test numbers in which the critical compression ratio was less than 75%, evaluation tests of carburized steel components were not conducted.

[Measurement test of number of pieces of Mn sulfide and oxide]

Samples were collected from each of the as-rolled members and the SA members of each test number described above. Specifically, as shown in FIG. 3, a sample was collected from the R/2 position in the radial direction from the central axis line C1 of the as-rolled member and the SA member. The size of the observation surface of the sample was L1×L2, where L1 was 10 mm and L2 was 5 mm. Further, the sample thickness L3 in the direction perpendicular to the observation surface was set to 5 mm. The normal line N of the observation surface was perpendicular to the central axis line C1, and the R/2 position corresponded to the central position of the observation surface.

The observation surface of the collected sample was mirror-polished, and 20 visual fields (evaluation area per one visual field was 100 m×100 μm) were randomly observed at a magnification of 1000 times using a scanning electron microscope (SEM) (20 visual fields for an as-rolled member, and 20 visual fields for an SA member).

Inclusions in each visual field were identified. For each of the identified inclusions, an energy dispersive X-ray spectroscopy (EDX) was used to discriminate Mn sulfide and oxide. Specifically, using the EDX, elemental analysis was performed at least two measurement points for each inclusion. Then, in each inclusion, the arithmetic average value of the element content obtained at each measurement point was defined as the content (mass %) of each element in the inclusion. When elemental analysis was performed at two measurement points in one inclusion, the arithmetic average value of Mn content, the arithmetic average value of S content, and the arithmetic average value of O content obtained at the two measurement points were defined as the Mn content (mass %), the S content (mass %), and the O content (mass %) in the inclusion. In the results of elemental analysis of the identified inclusions, when the Mn content was 10.0% or more, the S content was 10.0% or more, and the O content was less than 10.0%, it was presumed that the inclusion was Mn sulfide. Further, in the result of elemental analysis of the identified inclusion, when the O content was 10.0% more, it was presumed that the inclusion was oxide. The inclusions to be identified were inclusions having a circle-equivalent diameter of 0.5 μm or more. Moreover, the beam diameter of the EDX used to identify inclusions was 0.2 μm.

In each of the as-rolled member and the SA member, Mn sulfide and oxide having a circle-equivalent diameter of 0.5 μm or more were measured. The number of pieces of Mn sulfide per unit area (pieces/mm²) was determined based on the total number of pieces of Mn sulfide identified in each visual field and the total area of 20 visual fields. Further, the number of pieces of oxide per unit area (pieces/mm²) was determined based on the total number of pieces of oxide identified in each visual field and the total area of 20 visual fields.

Table 2 shows the number of pieces of Mn sulfide (pieces/mm²) in the as-rolled member and the number of pieces (pieces/mm²) of oxide in the as-rolled member. In each test number, the number of pieces of Mn sulfide in the SA member was the same as the number of pieces of Mn sulfide in the as-rolled member, and the number of pieces of oxide in the SA member was the same as the number of pieces of oxide in the as-rolled member.

[Production of carburized steel component]

Spheroidizing annealing was performed on the as-rolled member of each test number. Specifically, the as-rolled member was heated to 740° C. Thereafter, the as-rolled member was slowly cooled at a cooling rate of 8° C./hr until the temperature thereof reached 650° C. The steel material was air-cooled from 650° C. to room temperature to produce a spheroidizing annealed as-rolled member.

Test specimen was collected from the spheroidizing annealed as-rolled member. The test specimen was a round bar with a diameter of 29.5 mm and a length of 44 mm. The longitudinal direction of the test specimen was the same as the longitudinal direction of the as-rolled member.

Simulating cold forging, this test specimen was subjected to swaging at a compression ratio of 50% in a cold state. Swaging was performed at room temperature and an arresting die was used. The strain rate during swaging was set to 1/sec. The test specimen after swaging was subjected to gas carburization by a gas-converter method. In this gas carburization, the carbon potential was set to 0.8%, and the holding was performed at 950° C. for 5 hours, followed by holding at 850° C. for 0.5 hours. After gas carburization, oil quenching to 130° C. was performed as a finish heat treatment process. After quenching, tempering was performed at 150° C. for 90 minutes. Through the process described above, a test specimen simulating a carburized steel component was prepared from the as-rolled member.

For the SA member of each test number, a test specimen simulating a carburized steel component was prepared by the same method as for the above described as-rolled member. Specifically, test specimens were collected from the SA member of each test number. The test specimen was a round bar with a diameter of 29.5 mm and a length of 44 mm. The longitudinal direction of the test specimen was the same as the longitudinal direction of the SA member. Simulating cold forging, swaging at a compression ratio of 50% was performed on this test specimen in a cold state. Swaging was performed at room temperature and an arresting die was used. The strain rate during swaging was set to 1/sec. The test specimen after swaging was subjected to gas carburization by the gas-converter method. In this gas carburization, the carbon potential was set to 0.8%, and the holding was performed at 950° C. for 5 hours, followed by the holding at 850° C. for 0.5 hours. After gas carburization, oil quenching to 130° C. was performed as a finish heat treatment process. After quenching, tempering was performed at 150° C. for 90 minutes. Through the process described above, a test specimen simulating a carburized steel component was prepared from the SA member.

[Evaluation test of carburized steel component]

The following tests were conducted on the carburized layer and the core portion of thus produced test specimen (the as-rolled member, and the SA member) simulating a carburized steel component.

[Vickers hardness test of carburized layer]

On a cut cross section perpendicular to the longitudinal direction of the above described test specimen (as-rolled member, and SA member) simulating a carburized steel component of each test number, Vickers hardness at a position of 50 μm depth from the surface and Vickers hardness at a position of 0.4 mm depth from the surface were determined by Vickers hardness test in conformity with JIS Z 2244 (2009) using a micro Vickers hardness tester. The test force was 0.49 N. Vickers hardness HV was measured in 10 locations at a position of 50 μm depth, and an average value thereof was assumed as Vickers hardness HV at a position of 50 μm depth. Further, Vickers hardness HV was measured in 10 locations at a position of 0.4 mm depth, and an average value thereof was assumed as Vickers hardness HV at a position of 0.4 mm depth.

If the hardness at a position of 0.4 mm depth from the surface was 600 HV or more, it was determined that a carburized layer existed from the surface to at least 0.4 mm. Further, when Vickers hardness at a position of 50 μm depth from the surface was 650 to 1000 HV, it was judged that the hardness of the carburized layer of the carburized steel component was sufficient. Measurement results are shown in Table 3.

TABLE 3 Carburized steel component (as-rolled member→SA→carburized component) Carburized layer Core portion Martensite Presence/absence Hardness Hardness fraction Hardness of occurrence at position at position at position at position of coarse Fatigue Test Steel of 50 μm of 0.4 mm of 0.4 mm of 2.0 mm grains during strength Number Number depth (HV) depth (HV) depth (%) depth (HV) carburization ratio (%) 1 A 815 779 100.0 301 Absent 91 2 B1 805 781 100.0 325 Absent 88 3 C1 822 778 100.0 361 Absent 87 4 D 841 773 100.0 324 Absent 91 5 E 822 762 99.0 346 Absent 90 6 F 831 776 98.0 322 Absent 88 7 G 848 771 99.0 291 Absent 87 8 H 799 762 96.0 346 Absent 89 9 I 827 781 100.0 291 Absent 90 10 J 826 769 99.0 305 Absent 91 11 K 837 781 100.0 322 Absent 92 12 L — — — — — — 13 M 806 782 96.0 239 Absent 90 14 N 788 779 99.0 324 Absent 79 15 O 803 781 100.0 241 Absent 87 16 P — — — — — — 17 Q 796 756 96.0 243 Absent 88 18 R — — — — — — 19 S 811 545 96.0 328 Absent 86 20 T — — — — — — 21 U 789 769 98.0 347 Present 86 22 B2 807 778 100.0 324 Absent 67 23 C2 816 769 98.0 360 Absent 71 24 B3 842 774 100.0 325 Absent 77 25 C3 788 781 100.0 358 Absent 81 26 B1 — — — — — — 27 C1 — — — — — — 28 V 800 780 100.0 323 Absent 86 29 W — — — — — — 30 X — — — — — — Carburized steel component (SA member→carburized steel component) Carburized layer Core portion Martensite Presence/absence Hardness Hardness fraction Hardness of occurrence at position at position at position at position of coarse Fatigue Test of 50 μm of 0.4 mm of 0.4 mm of 2.0 mm grains during strength Number depth (HV) depth (HV) depth (%) depth (HV) carburization ratio (%) 1 816 778 98.0 302 Absent 90 2 804 783 100.0 322 Absent 89 3 823 779 99.0 359 Absent 88 4 839 775 100.0 326 Absent 90 5 821 761 100.0 344 Absent 91 6 830 778 99.0 321 Absent 87 7 847 772 98.0 292 Absent 88 8 800 763 97.0 345 Absent 90 9 826 779 99.0 289 Absent 91 10 828 767 100.0 304 Absent 90 11 834 783 100.0 319 Absent 91 12 — — — — — — 13 809 781 98.0 238 Absent 89 14 781 776 98.0 321 Absent 80 15 801 783 99.0 245 Absent 88 16 — — — — — — 17 798 755 97.0 242 Absent 87 18 — — — — — — 19 810 548 95.0 326 Absent 85 20 — — — — — — 21 781 771 99.0 344 Present 88 22 804 776 98.0 322 Absent 66 23 817 770 100.0 358 Absent 70 24 841 773 98.0 324 Absent 78 25 792 782 100.0 356 Absent 80 26 798 777 98.0 320 Absent 87 27 816 776 100.0 344 Absent 90 28 802 781 100.0 321 Absent 86 29 — — — — — — 30 — — — — — —

[Microstructure observation test of carburized layer]

The microstructure at a position of 0.4 mm depth from the surface of the above described test specimen (as-rolled member, and SA member) simulating a carburized steel component was determined by the following method. A sample which included, on the surface thereof, a position of 0.4 mm depth from the surface of the test specimen was collected. The surface of the sample was etched with a Picral solution. Of the surface after etching, any three visual fields were observed with a secondary electron image by using the SEM. The area of each visual field was 400 μm² (magnification of 5000 times). Martensite and bainite (including tempered martensite and tempered bainite), ferrite, pearlite, and cementite could be distinguished from contrast. The total area of martensite in the three visual fields of the sample of a position of 0.4 mm depth was determined. The ratio of the determined total area of martensite to the total area of the three visual fields was defined as the area fraction (%) of martensite at a position of 0.4 μm depth.

[Vickers hardness test and chemical composition identification of core portion]

Vickers hardness and chemical composition of the core portion of the test specimen simulating a carburized steel component were measured by the following method. On a cut cross section perpendicular to the longitudinal direction of a carburized steel component, Vickers hardness at a position of 2.0 mm depth from the surface was determined by Vickers hardness test in conformity with JIS Z 2244 (2009) using a micro Vickers hardness tester. The test force was 0.49 N. The measurement was performed 10 times at a position of 2.0 mm depth, and an average value thereof was assumed as Vickers hardness (HV) at a position of 2.0 mm depth from the surface. The obtained Vickers hardness is shown in Table 3. When the Vickers hardness at a position of 0.2 mm depth was 250 to 500 HV, it was judged that the core portion hardness was sufficient and acceptable. When Vickers hardness at a position of 0.4 mm depth was 600 HV or more and Vickers hardness at a position of 2.0 mm was less than 600 HV, it was judged that the carburized layer had an effective hardened layer of 0.4 to less than 2.0 mm. That is, in this case, the position of 2.0 mm depth from the surface was presumed to be the core portion.

[Presence or absence of coarse grains in carburized steel component]

As for the core portion of a test specimen (as-rolled member and SA member) simulating a carburized steel component, prior-austenite grains were observed at a position of 2 mm depth from the surface. Specifically, a cut cross section perpendicular to the longitudinal direction of the carburized steel component was used as the observation surface. After being mirror polished, the observation surface was subjected to etching with a saturated aqueous solution of picric acid. A visual field (300 μm×300 μm) including a position of 2 mm depth from the surface of the etched observation surface was observed with an optical microscope (a magnification of 400 times) to identify prior-austenite grains. For the identified prior-austenite grains, the grain size of each prior-austenite grain was determined by a circle-equivalent diameter (μm) in conformity with JAS G 0551 (2013). When even one grain having a circle-equivalent diameter of more than the circle-equivalent diameter (88.4 μm) corresponding to the grain size number 4 specified in JIS described above was present among the prior-austenite grains, it was judged that “coarse grains occurred”. The judgment results are shown in Table 3.

[Fatigue strength evaluation test of carburized steel component after joining]

The as-rolled member and SA member of each test number were machined to prepare a round bar having a diameter of 20 mm and a length of 150 mm. Using this round bar (the as-rolled member, and the SA member), a basic fatigue test specimen and a joint fatigue test specimen were prepared. Before making the following basic fatigue test specimen and joint fatigue test specimen, spheroidizing annealing was performed on the as-rolled member under the same conditions as described above, and thereafter machining was performed to prepare a round bar having a diameter of 20 mm and a length of 150 mm.

The basic fatigue test specimen was prepared by the following method. An Ono-type rotary bending fatigue test specimen having an evaluation portion diameter of 8 mm and a parallel portion length of 15 mm was prepared from the central portion of the cross section of a round bar having a diameter of 20 mm and a length of 150 mm. This test specimen was used as a basic fatigue test specimen. The longitudinal direction of the basic fatigue test specimen was the same as the longitudinal direction of the round bar.

The joint fatigue test specimen was prepared by the following method. Round bars of the same test specimen number, which each had a diameter of 20 mm and a length of 150 mm, were butted against each other to prepare a joined round bar under the following friction pressure welding conditions.

Friction pressure welding condition:

Friction pressure: 100 MPa,

Friction time: 5 seconds,

Upset pressure (force to pressurize the joined portion from both end portions of the round bar): 200 MPa,

Upset time (time to pressurize the joined portion: 5 seconds,

Rotational speed: 2000 rpm, and

Burn-off length: 5 to 12 mm.

An Ono-type rotary bending fatigue test specimen having an evaluation portion diameter of 8 mm and a parallel portion length of 15.0 mm was prepared from the central portion of the cross section of the joined round bar and used as a pressure welding fatigue test specimen. In the pressure welding fatigue test specimen, the central portion in the longitudinal direction of the parallel portion was used as the joining surface. The longitudinal direction of the joint fatigue test specimen was the same as the longitudinal direction of the round bar.

The following carburizing and quenching treatment was performed on the basic fatigue test specimen and the joint fatigue test specimen to obtain carburized steel components (test specimens using as-rolled member after spheroidizing heat treatment, and test specimens using SA member). In the carburizing and quenching treatment, gas carburizing by a gas-converter method was performed. Specifically, the carbon potential was set to 0.8%, and the furnace was held at 950° C. for 5 hours. Thereafter, the furnace was held at 850° C. for 0.5 hours at the same carbon potential. Thereafter, the specimens were immersed in oil at 130° C. to perform oil quenching. After the oil quenching, tempering for holding at 150° C. for 90 minutes was performed. By the method described so far, an Ono type rotary bending fatigue test specimens (basic fatigue test specimen, and joint fatigue test specimen) simulating a carburized steel component were prepared.

The Ono-type rotary bending fatigue test was conducted on the prepared basic fatigue test specimens and joint fatigue test specimens. Specifically, using each of the above described Ono-type rotary bending fatigue test specimens (basic fatigue test specimen, and joint fatigue test specimen), Ono-type rotary bending fatigue test conforming to JIS Z 2274 (1978) was conducted at room temperature in the atmosphere. With a rotational speed as 3000 rpm, and a stress ratio R as −1, the maximum stress at which no breakage occurred after the number of repetitions of stress loading was 1×10⁷ cycles was defined as the fatigue strength (MPa).

The ratio (%) of the fatigue strength (MPa) of the joint fatigue test specimen to the fatigue strength (MPa) of the basic fatigue test specimen was defined as the fatigue strength ratio. That is, the fatigue strength ratio was defined by the following formula.

Fatigue strength ratio (%)=fatigue strength of joint fatigue test specimen/fatigue strength of basic fatigue test specimen×100

The obtained fatigue strength ratio is shown in Table 3. When the fatigue strength ratio was 85% or more, it was judged that excellent fatigue strength was achieved even after joining.

[Test results]

The test results are shown in Tables 2 and 3. Referring to Tables 2 and 3, the chemical compositions of Test Numbers 1 to 11 and 28 were appropriate and satisfied Formulae (1) to (3). In addition, the steelmaking conditions were appropriate. Moreover, the cooling rate in the cooling process was also appropriate. Therefore, the number of pieces of MnS in the as-rolled member and the SA member was 70.0 pieces/mm² or less, and the number of pieces of oxide was 25.0 pieces/mm² or less. Further, in the as-rolled member, the bainite area fraction in at least the outer layer region from the surface to 0.1R depth was 95.0% or more (“Y” in the “microstructure of the as-rolled member” column in Table 2), and in the SA member, the microstructure of at least the outer layer region from the surface to 0.1R depth is composed of ferrite and cementite, and the spheroidization ratio of cementite in the outer layer region was 90.0% or more (“Y” in the “microstructure of the SA member” column in Table 2). As a result, the critical compression ratio of the as-rolled member was 50% or more, and the critical compression ratio of the SA member was 75% or more so that an excellent critical compression ratio was exhibited.

Further, referring to Table 3, in a test specimen simulating a carburized steel component produced by using an as-rolled member, Vickers hardness at a position of 50 m depth was 650 to 1000 HV, the martensite area fraction at a position of 0.4 mm depth was 90.0% or more, and Vickers hardness at a position of 0.4 mm depth was 600 to 900 HV or more. Further, Vickers hardness at a position of 2.0 mm depth from the surface was 250 to 500 HV, and an effective hardened layer depth of the carburized layer was 0.4 to less than 2.0 mm. Furthermore, prior-austenite grain boundaries were not coarsened in the core portion. Further, the fatigue strength ratios based on the joint fatigue test specimen and the basic fatigue test specimen were all as high as 85% or more, and exhibited excellent fatigue strength even when joined and even after welding.

On the other hand, in Test Number 12, the C content was too high. Therefore, the critical compression ratio of the as-rolled member was less than 50%. Further, the critical compression ratio of the SA member was less than 75%, and a sufficient critical compression ratio could not be obtained.

In Test Number 13, the C content was too low. Therefore, sufficient hardness could not be obtained in the core portion of the test specimen simulating a carburized steel component.

In Test Number 14, the oxygen content was too high. Therefore, the number of pieces of oxide was too large. As a result, the fatigue strength ratio based on the joint fatigue test specimen and the basic fatigue test specimen of the as-rolled member and the SA member was as low as less than 85%, and the fatigue strength after welding was low.

In Test Number 15, the N content was too high. Therefore, the solute B could not be secured, and the core portion hardness was too low.

In Test Numbers 16 and 29, F1 was more than the upper limit of Formula (1). Therefore, the critical upsetting ratios of the as-rolled member and the SA member were low.

In Test Number 17, F1 was less than the lower limit of Formula (1). Therefore, the core portion hardness of the carburized component of the as-rolled member, and the core portion hardness of the carburized steel components of the SA member were too low.

In Test Numbers 18 and 30, F2 was more than the upper limit of Formula (2). Therefore, the critical upsetting ratios of the as-rolled member and the SA member were low.

In Test Number 19, F2 was less than the lower limit of Formula (2). Therefore, in the carburized component of the as-rolled member and the carburized steel component of the SA member, hardness at a position of 0.4 mm depth was too low.

In Test Number 20, F3 was more than the upper limit of Formula (3). Therefore, the critical upsetting ratio of steel material (as-rolled member and SA member) was low.

In Test Number 21, F3 was less than the lower limit of Formula (3). Therefore, some of prior-austenite grains were coarsened in the core portion of the carburized component of the as-rolled member and the core portion of the carburized steel component of the SA member.

In Test Numbers 22 and 23, regarding the molten steel in the ladle after the secondary smelting, the holding time ts at a temperature of 1500 to 1600° C. was less than 2.0 times the uniform mixing time τ. Therefore, in the as-rolled member and the SA member, the number of pieces of MnS was more than 70.0/mm² and the number of pieces of oxide was more than 25.0/mm². As a result, the fatigue strength ratio was as low as less than 85% in the joint fatigue test specimen simulating a carburized component of the as-rolled member and a carburized steel component of the SA member.

In Test Numbers 24 and 25, Si was added before adding Al in the smelting process. Therefore, the number of pieces of oxide in the as-rolled member and the SA member was more than 25.0/mm². As a result, the fatigue strength ratio was as low as less than 85% in the joint fatigue test specimen simulating a carburized component of the as-rolled member and a carburized steel component of the SA member.

In Test Numbers 26 and 27, the average cooling rate at 800 to 500 in the slow cooling process after hot rolling was too fast. Therefore, the microstructure of the steel material of as-rolled member was the microstructure mainly composed of martensite. As a result, the critical compression ratio of the as-rolled member was less than 50%. However, in the microstructure of the SA member, the microstructure of at least the outer layer region from the surface to 0.1R depth was composed of ferrite and cementite, and the spheroidization ratio of cementite in the outer layer region was 90.0% or more. Therefore, the critical compression ratio of the SA member was 75% or more. Vickers hardness of the carburized layer of a carburized steel component of the SA material was appropriate, and the martensite fraction at a position of 0.4 mm depth was 90.0% or more. Furthermore, the core portion hardness and chemical composition were also appropriate, and the prior-austenite grain size was not coarsened. Further, in the joint fatigue test specimen, the fatigue strength ratio was as high as 85% or more, and even when joined, it showed excellent fatigue strength.

So far, embodiments of the present invention have been described. However, the above described embodiments are merely examples for practicing the present invention. Therefore, the present invention can be practiced by appropriately modifying the above described embodiments within a range not departing from the spirit thereof, without being limited to the above described embodiments. 

1. A steel material consisting of: in mass %, C: 0.09 to 0.16%, Si: 0.01 to 0.50%, Mn: 0.40 to 0.60%, P: 0.030% or less, S: 0.025% or less, Cr: 0.90 to 2.00%, Mo: 0.10 to 0.40%, Al: 0.005 to 0.030%, Ti: 0.010 to less than 0.050%, Nb: 0.010 to 0.030%, N: 0.0080% or less, O: 0.0030% or less, B: 0.0003 to 0.0030%, Ca: 0.0005 to 0.0050%, Cu: 0 to 0.50%, Ni: 0 to 0.30%, and V: 0 to 0.10%, with the balance being Fe and impurities, and the steel material satisfying Formula (1) to Formula (3), wherein in a cross section parallel to an axial direction of the steel material, an amount of Mn sulfide having, in mass %, a Mn content of 10.0% or more, a S content of 10.0% or more, and an O content of less than 10.0% is 70.0 pieces/mm² or less, and an amount of oxide having an O content of 10.0% or more is 25.0 pieces/mm² or less: 0.140<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235  (1) 13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0   (2) 0.004<Ti—N×(48114)<0.030  (3) where, each element symbol of the Formulae (1) to (3) is substituted by the content (mass %) of the corresponding element, and if the corresponding element is not contained, the element symbol is substituted by “0”.
 2. The steel material according to claim 1, wherein when a radius in a cross section perpendicular to an axial direction of the steel material is defined as R (mm), in a microstructure in the cross section perpendicular to the axial direction of the steel material, an area fraction of bainite in at least an outer layer region from a surface to 0.1R depth is 95.0% or more.
 3. The steel material according to claim 1, wherein when a radius in a cross section perpendicular to an axial direction of the steel material is defined as R (mm), in a microstructure in the cross section perpendicular to the axial direction of the steel material, at least an outer layer region from the surface to 0.1R depth is composed of ferrite and cementite, and a spheroidization ratio of the cementite in the outer layer region is 90.0% or more.
 4. The steel material according to claim 1, containing, one or more elements selected from the group consisting of: in mass %, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, and V: 0.01 to 0.10%.
 5. The steel material according to claim 2, containing, one or more elements selected from the group consisting of: in mass %, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, and V: 0.01 to 0.10%.
 6. The steel material according to claim 3, containing, one or more elements selected from the group consisting of: in mass %, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, and V: 0.01 to 0.10%. 